Recorded Seminars

Below are recordings of some of the seminars hosted by FTPI. Also listed are FTPI faculty interviews and seminars where permissions allow.


High Energy Theory Seminars | Condensed Matter Theory Seminars

FTPI Seminars | FTPI Faculty Interviews & Seminars

Larkin Award in Theoretical Physics


Cristina Mondino delivering the High Energy Seminar

High Energy Theory Seminars

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2/9/24: Presented by Dr. Yichul Choi from Stony Brook University in New York. The title of his talk is, “Quantization of Axion-Gauge Couplings and Generalized Symmetries.”

Abstract: A rarely discussed fact about the Standard Model (SM) is that even though we know that it enjoys the su(3) x su(2) x u(1) gauge symmetry, there are 4 distinct possible global forms of the SM gauge group, sharing the same Lie algebra, which are all consistent with the current experiments at colliders. We show that when an axion couples to the SM, the precise global form of the gauge group becomes important and affects the quantization conditions imposed on axion-gauge couplings. Using the quantization conditions, we derive a constraint put on the effective axion-photon coupling. In the latter part of the talk, we will discuss generalized global symmetries in the presence of an axion and their implications on the monopole mass and axion string tension. Based on and

2/2/24: Presented by Dr. Cari Cesarotti from the Massachusetts Institute of Technology (MIT). The title of her talk is, “Physics Potential at a Future Muon Collider.”

Abstract: A clear outcome of Snowmass 2021 and now the US P5 report was the community support for R&D towards a future muon collider. In this talk we will discuss the general physics program that becomes available to the community during the construction and completion of the future collider. We will review not only the main challenges and advantages of such a collider compared to other possibilities, but also the projected reach of several specific models. Additionally, we consider the physics possibilities at the necessary demonstrator facilities along the way. For example, a beam dump would be an economical and effective way to increase the discovery potential of the collider complex in a complementary regime.

1/19/24: Presented by Dr. Marios Galanis from the Perimeter Institute in Canada. The title of his talk is, “Precision Astrometry and AGN Physics with Intensity Interferometry.”

Abstract: Pioneered in the 1950s by Hanbury Brown and Twiss, intensity interferometry refers to the correlation of light intensities incident on two telescopes. As its name suggests, it relies only on photon counting, allowing for interferometry with arbitrarily long baselines in optical wavelengths. Its chief drawbacks are the need for very bright sources and a limited field-of-view, and is thus restricted to date to the study of nearby stellar morphologies. The goal of this talk is twofold: explain how these two disadvantages can be overcome, and present entirely new science cases that this will make possible. First, I will show how recent advances in photodetection technology and spectroscopy will allow us to target the fainter morphologies of Active Galactic Nuclei (AGNs) and establish a novel geometric method to measure the Hubble constant. Next, I will present a new type of telescope array with an optical-path modification, the Extended-Path Intensity Correlation (EPIC), which expands the field-of-view by several orders of magnitude. EPIC will allow for Astrometry at an unprecedented level of precision: bright sources whose separation is as much as an arcsecond can be measured with sub-microarcsecond light-centroiding precision. I will end with a discussion of EPIC’s numerous applications.

12/15/23: Presented by Dr. Andrew Eberhardt from the KAVLI Institute for the Physics and Mathematics of the Universe (IPMU) in Japan. The title of his talk is, “The Classical Field Approximation in ULDM.”

Abstract: Given our current understanding of the universe, about a fourth of the cosmic energy density must be "dark" matter. And while the abundance and interaction strength are well constrained, the particle nature is entirely unknown. A global effort combines simulations, observation, and theory to search approximately 100 orders of magnitude of mass parameter space in an attempt to identify the dark matter particle. At the lowest mass end, around 1e-19 eV and below, the particle is so light that it manifests quantum effects on galactic scales. In this regime, simulations of structure formation provide some of the strongest constraints on the dark matter mass. These simulations use the classical field approximation to make numerical results computationally feasible. Recently, there has been debate in the community as to whether this approximation is sufficient to describe the behavior of ultra light dark matter on all length and time scales relevant to current constraints. We critically examine this claim by developing some of the largest and most accurate quantum field simulations of ultra light dark matter. Our work identifies the scales on which this approximation breaks down and the effect of quantum corrections. We find that corrections grow exponentially and impact the density profile predictions of this model.

11/17/23: Presented by Dr. Stefania Gori from the University of California, Santa Cruz. The title of her talk is, “New ALP Signatures at High Intensity Experiments.”

Abstract: Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons and pions benefit from their small total decay widths and the availability of very large datasets. In this talk, we first give an overview of present and future light meson factories (e.g., PIONEER and NA62) and proton beam dump experiments that produce the largest samples of light mesons (e.g., DarkQuest experiment). Second, we discuss new opportunities to search for axion-like particles (ALPs) at these experiments, focusing on a few minimal ALP models, including ALPs coupled to leptons in an isospin-violating way.

11/10/23: Presented by Dr. Tao Xu from the University of Oklahoma. The title of his talk is, “Phenomenology of Black Holes Across the Mass Range.”

Abstract: In recent years, there has been a growing interest in black holes that originate from non-astrophysical mechanisms. These hypothetical black holes can vary in mass, covering a spectrum of possibilities from particle physics scales to masses significantly greater than astrophysical objects. The searches for their existence across different mass ranges have motivated studies into black hole physics from a phenomenology perspective. In this seminar, I will discuss two interesting mass windows: black holes lighter than a kiloton, which can impact early universe cosmology, and black holes falling within the asteroid-mass range, potentially contributing to the dark matter abundance. Moreover, I will highlight a novel avenue for probing Beyond Standard Model physics through black holes.

11/3/23: Presented by Dr. Peizhi Du from Rutgers University in New Jersey. The title of his talk is, “New Semiconductor Devices for Dark Matter Detection.”

Abstract: Dark matter remains a fundamental mystery in particle physics, and extensive efforts have been made to detect it in labs. In this talk, I will summarize the recent progress in sub-GeV dark matter detection with semiconductors and introduce our new idea to probe sub-MeV dark matter with doped semiconductors. I will also discuss the origin of the unexplored backgrounds in current detectors and demonstrate our new proposed device, called Dual-sided CCD, can significantly reduce some those backgrounds. 

10/27/23: Presented by Dr. Rajan Gupta from the Los Alamos National Laboratory (LANL) in New Mexico. The title of his talk is, “Contributions of CP Violating Operators to the Neutron/Proton EDM using Lattice QCD.”

Abstract: One of the profound mysteries of nature is the lack of matter-antimatter symmetry in the universe, i.e., the almost total absence of antibaryons. One of the conditions necessary to generate this asymmetry is the violation of charge-conjugation-parity (CP) symmetry. Every interaction that violates CP in theories beyond the standard model (BSM) also contributes to the neutron electric dipole moment (nEDM). Thus, a value (or bound) on the nEDM provides constraints on possible BSM theories. I will describe the status of the calculations of the contributions of the quark EDM, quark chromo EDM, Theta and Weinberg terms to the nEDM that are being carried out by the Los Alamos collaboration. 

10/20/23: Presented by Dr. Soubhik Kumar from New York Univerity. The title of his talk is, “Rare Processes during Inflation: CMB Hotspotsand Early Galaxies.”

Abstract: Superheavy particles can be produced during the inflationary era of the primordial Universe. Owing to their large mass, they are produced rarely. However, once produced, they can give rise to significant perturbations in the spacetime metric around their locations. Such perturbations eventually would give rise to overdensities in the primordial plasma. I will describe two late-time signatures of such overdensities: (i) hotspots on the cosmic microwave background, and (ii) anomalously massive early galaxies. I will also demonstrate that novel search strategies are required to look for such signatures and those would constitute one of the unique probes of particle physics at very high energies. 

10/13/23: Presented by Dr. Ryan Plestid from the California Institute of Technology (Caltech). The title of his talk is, “The Effective Theory of Coulomb Corrections.”

Abstract: When electrically charged heavy particles participate in a scattering event, large Coulomb corrections must often be resummed (or treated to high perturbative orders). Historically, this has been achieved by noting that the leading-Z series can be captured using a background field model, however this cannot always be extended to higher orders in $\alpha$. In this talk I will explain how to connect modern notions of effective field theory and the method of regions to historical "wavefunction solutions'' to the resummation of Coulomb corrections. Applications to the extraction of $|V_ud|$ from superallowed beta decays will be discussed.

10/6/23: Presented by Dr. Maximilian Ruhdorfer from Cornell University in New York. The title of his talk is, “On the Dynamical Origin of the η′ Potential and the Axion Mass.”

Abstract: The standard lore posits that pure QCD dynamics generates a confining potential with a branched structure as a function of the θ angle. This same potential also largely determines the properties of the η′, the (would-be) Goldstone boson associated with the anomalous axial U(1) symmetry of QCD, once fermions are included. In this talk I will describe how this picture can be tested by examining a supersymmetric extension of QCD with a small amount of supersymmetry breaking generated via anomaly mediation. I will demonstrate that for pure SU(N) QCD without flavors there are N branches generated by gaugino condensation. However, once quarks are introduced, flavor effects qualitatively change the strong dynamics, leading to |N-F| branches for F flavors. I will also show that for F=N-1,N,N+1 there are no branches and the entire potential is consistent with being a one-instanton effect. Finally I will comment on spontaneous CP breaking in the theory with one flavor. This talk will be based on arXiv:2307.04809 and work in progress.

9/29/23: Presented by Dr. Mukul Sholapurkar from the University of California, San Diego. The title of his talk is, “Anharmonic Effects in Nuclear Recoils from Sub-GeV Dark Matter.”

Abstract: Direct detection experiments are looking for nuclear recoils from scattering of sub-GeV dark matter (DM) in crystals, and have reached thresholds as low as ~ 10 eV. Future experiments are aiming for even lower thresholds. At such low energies, the free nuclear recoil prescription breaks down, and the relevant final states are phonons in the crystal. Scattering rates into multi-phonons have already been computed for a harmonic crystal. However, crystals typically exhibit some anharmonicity, which can significantly impact scattering rates in certain kinematic regimes. In this seminar, I will present estimates of the impact of anharmonic effects on scattering rates for DM, focusing on the DM mass range ~ 1-10 MeV, where the details of multi-phonon production are most important. I will talk about both analytic and numerical estimates using a simple 1D model of a single atomic potential.

9/22/23: Presented by Dr. Quentin Bonnefoy from the University of California, Berkeley. The title of his talk is, “A Colorful Mirror Solution to the Strong CP Problem.”

Abstract: Theories which spontaneously break spacetime parity can solve the strong CP problem. They usually have few free parameters and are therefore very predictive, but their landscape remains quite unexplored. I will present a construction based on a complete mirror copy of the standard model, linked to our world by colored portal fields. Those induce the partial spontaneous breaking of the color groups, which yields a vanishing theta angle at low energies. The lightest BSM fields could be found at colliders, and are either colored (pseudo-Goldstone or vector) bosons, or some of the vectorlike fermions predicted by parity. The lightest of the latter can actually play the role of thermal dark matter in our model, unlike what was previously found in similar constructions.

9/15/23: Presented by Dr. Dawid Brzeminski from the University of Maryland. The title of his talk is, “Dynamical Equilibration of Dark Matter and Baryon Energy Densities.”

Abstract: The near equality of the dark matter and baryon energy densities is a remarkable coincidence, especially when one realizes that the baryon mass is exponentially sensitive to UV parameters in the form of dimensional transmutations. We explore a new dynamical mechanism, where in the presence of an arbitrary number density of baryons and dark matter, a scalar adjusts the masses of dark matter and baryons until the two energy densities are comparable. In this manner, the coincidence is explained regardless of the microscopic identity of dark matter and how it was produced. This new scalar causes a variety of experimental effects such as a new force and a (dark) matter density dependent proton mass.

9/8/23: Presented by Dr. Anirudh Prabhu from Princeton University in New Jersey. The title of his talk is, “Axions in High-energy Astrophysical Plasmas.”

Abstract: Axions are a well-motivated extension to the Standard Model and among the best candidates to explain dark matter. Their detection is made difficult by the fact that they couple very weakly to particles in the Standard Model. High-energy astrophysical settings host extreme conditions wherein axions may be produced in great abundance. In this talk, I will discuss axion production in the highly magnetized plasma surrounding compact objects, particularly neutron stars. Once produced, axions may re-convert to photons, leading to anomalous emission. Radio observations of nearby pulsars can place stringent constraints on the axion-photon coupling that improve upon existing bounds by orders of magnitude for a range of axion masses. Additionally, axion production around neutron stars may provide an explanation for the mysterious Fast Radio Bursts observed by several radio missions. Finally, I will comment on the detection prospects for these neutron star-sourced axions in proposed axion detection experiments.

4/28/23: Presented by Dr. Markus Luty from the University of California, Davis. The title of his talk is, “Blowing in the Dark Matter Wind.”

Abstract: If dark matter interacts strongly with ordinary matter, it would exert a mechanical pressure of order 10^{14} Newtons/cm^2 due to the fact that the dark matter wind is "blowing" at 250 km/sec. Interestingly, this force is at the level of the most precise 5th force experiments. The possibility of such interactions between dark matter and ordinary matter is ruled out for particle-like dark matter, but is compatible with all constraints in certain models of field-like dark matter. The physical mechanism is a dark matter version of the Meissner effect in superconductors. I will argue that the force can be detected with a relatively simple modification of existing torsion balance experiments.

4/21/23: Presented by Dr. Amalia Madden from the Perimeter Institute in Canada. The title of her talk is, “The Piezoaxionic Effect.”

Abstract: The QCD axion is one of the best motivated extensions to the Standard Model, providing a solution to the strong CP problem as well as being an excellent dark matter candidate. In this talk, I will demonstrate how the spontaneous violation of parity within piezoelectric materials can induce several new experimental observables for the QCD axion. The first effect, termed "the piezoaxionic effect", involves the generation of an oscillating mechanical stress within a piezoelectric crystal due to the presence of QCD axion dark matter. Our proposed experimental setup could probe this effect for axion masses between 10^-11 eV and 10^-7 eV. The second effect, termed "the ferroaxionic effect", involves a QCD axion-mediated force that can be sourced by a piezoelectric crystal. This force can be detected via nuclear magnetic resonance and has sensitivity to axion masses between 10^-5 eV and 10^-2 eV. Together, these new observables could be used in the near future to probe two challenging mass ranges for most axion detection concepts.

4/14/23: Presented by Dr. Glennys Farrar from New York University. The title of her talk is, “Missed Resonances from Precision d-d- ->mu+mu- data:  Evidence for Sexaquark Dark Matter?”

Abstract: Last year, I realized that production of Sexaquarks in e+e- collisions would lead to final states in e+e- -> hadrons that would have been missed, given the event selection requirements of experiments to date. Depending on the production level, this could explain or partly explain the g-2 and lattice QCD hadronic vacuum polarization discrepancies with the values derived from R_had (e+e- -> hadrons) data. In this talk I will review that, then report on a recent examination of high-precision BESIII data on e+e- -> mu+mu- , whose energy dependence is sensitive to R_had. Fits to R_mumu using Rhad data have a CL of less than 10^-11, but allowing for 1 or 2 resonances missed in R_had gives an excellent fit, with the resonances having a significance of 8.2 and 6.5 sigma. I will give a “report card” on the successes and potential challenges of Sexaquark Dark Matter.

4/7/23: Presented by Dr. Ignatios Antoniadis from the Jussieu in France. The title of his talk is, “Swampland Program, Extra Dimensions and Symmetry Breaking.”

Abstract: I will argue on the possibility that the smallness of some physical parameters signal a universe corresponding to a large distance corner in the string landscape of vacua. Such parameters can be the scales of dark energy and supersymmetry breaking, leading to a generalization of the dark dimension proposal. I will discuss the theoretical framework and some of its main physical implications to particle physics and cosmology.

3/24/23: Presented by Dr. Leonardo Rastelli from Stony Brook University in New York. The title of his talk is, “Bootstrapping Pions at large N.”

Abstract: We formulate the problem of carving out the space of large N confining gauge theories in a modern bootstrap framework. The ultimate goal is cornering large N QCD and other physically interesting theories. We derive universal bounds on the effective field theory of massless pions by imposing the full set of positivity constraints that follow from 2 to 2 scattering. The exclusion boundary exhibits a sharp kink and we critically examine the possibility that large N QCD may sit there. Time permitting, I may also preview some work in progress on incorporating the chiral anomaly and on extending the setup to include external rho mesons.

3/17/23: Presented by Dr. Stephen Martin from Northern Illinois University. The title of his talk is, “High-quality Axions in Supersymmetry.”

Abstract: A class of solutions to the mu problem in supersymmetry feature also solve the strong CP problem, by introducing an axion with decay constant of order the geometric mean of the Planck and TeV scales, consistent with astrophysical limits. I discuss minimal models of this type with two gauge-singlet fields that break a Peccei-Quinn symmetry, and extensions with extra vectorlike quark and lepton supermultiplets. There are many possible anomaly-free discrete symmetries that protect the Peccei-Quinn symmetry to sufficiently high order to solve the strong CP problem. Models of this type that are automatically free of the domain wall problem require at least one pair of strongly interacting vectorlike multiplets with mass at the intermediate scale, and predict axion couplings that are greatly enhanced, putting them within reach of proposed axion searches.

2/24/23: Presented by Dr. Seth Koren from the University of Chicago in Illinois. The title of his talk is, “Putting Generalized Symmetries to Work for Particle Physics.”

Abstract: Over the past decade, field theorists have developed a novel framework for thinking about global symmetries which has enormously generalized our understanding thereof. Quite recently, my collaborators and I have established positively that past being a useful formal tool, such generalized symmetries are indeed present in models that we care about as particle physicists---and furthermore understanding them can lead to new insights into these models. As a first example, the Standard Model itself has a 'higher-group' symmetry intertwining flavor and hypercharge, and I will discuss how this symmetry controls the structure of unification. Going Beyond, I will show that the identification of a 'non-invertible' symmetry of Z' models of L_µ - L_τ reveals the existence of simple UV completions thereof where the scale of neutrino masses is exponentially suppressed from that of charged leptons.

2/10/23: Presented by Dr. Asher Berlin from the Fermi National Accelerator Laboratory (Fermilab) in Illinois. The title of his talk is, “Electromagnetic Signals of High-Frequency Gravitational Waves.”

Abstract: There is strong motivation to extend the observable frequency range of gravitational waves (GWs) beyond the Hz - kHz regime already probed by LIGO and Virgo. In particular, higher-frequency GWs can give rise to new classes of electromagnetic signals that can be searched for with small-scale detectors. A gauge-invariant description shows that existing experiments designed for the detection of axion dark matter only need to reanalyze existing data to search for such signals. I will also discuss how electromagnetic cavities can operate as exquisite mechanical to electromagnetic converters, enabling a broader search across orders of magnitude of unexplored parameter space.

2/3/23: Presented by Dr. Andreas Helset from the California Institute of Technology (Caltech). The title of his talk is, “Geometry in Particle Scattering.”

Abstract: One central property of the S-matrix is its invariance under field redefinitions. I will discuss how the geometry of field space makes this invariance manifest. This geometric formulation also has practical consequences. The scattering amplitudes  satisfy a geometric soft theorem. Also, scattering  amplitudes and the renormalization group equations for a theory of scalars and gauge bosons only depend on geometric quantities. I will apply these  geometric techniques to calculate parts of the renormalization group equations for the Standard  Model Effective Field Theory including operators up to mass dimension eight.

1/27/23: Presented by Dr. Paul Wiegmann from the University of Chicago in Illinois. The title of his talk is, “Chiral anomaly in classical Euler hydrodynamic."


12/9/22: Presented by Dr. Antonio Riotto from Geneva University in Switzerland. The title of his talk is, “Primordial Black Holes in the Era of Gravitational Wave Astronomy.”

Abstract: The discovery of a gravitational wave signal coming from the merger of two black holes by the LIGO/Virgo collaboration has initiated the new era of gravitational wave astronomy. Primordial black holes were immediately suggested to be responsible for such a signal thus initiating a flourish research activity on the subject on which we will report the state of the art.

12/2/22: Presented by Dr. Ethan Neil from the University of Colorado, Boulder. The title of his talk is, “Lattice Insights for Composite New Physics.”

Abstract: Lattice gauge theory has been an invaluable tool in calculating predictions in QCD from first principles. It can also be used as a numerical laboratory to explore the properties of other strongly-interacting gauge theories that may appear in models of new physics beyond the Standard Model.  In this talk, I describe a selection of lattice results both focused on individual theories such as a composite Higgs model, as well as on qualitative properties in the larger space of strongly-coupled theories.

11/18/22: Presented by Dr. Nina Coyle from the University of Chicago in Illinois. The title of her talk is, “Neutrino-nucleus Interaction Modeling and New Physics Searches.”

Abstract: Accurate neutrino-nucleus interaction modeling is an essential requirement for the success of the accelerator-based neutrino program. As no satisfactory description of cross sections exists, experiments tune neutrino-nucleus interactions to data to mitigate mis-modeling. In this talk, I will discuss how the interplay between near detector tuning and cross section mis-modeling affects new physics searches. I will present two illustrative new physics scenarios, light sterile neutrinos and neutrinophilic scalars, present the relevant experimental signatures with and without tuning, and discuss the prospects of identifying new physics.

11/11/22: Presented by Dr. Sergey Sibiryakov from the Perimeter Institute in Canada. The title of his talk is, “Condensation and Evaporation of Boson Stars.”

Abstract: Axion-like particles, including the QCD axion, are well-motivated dark matter candidates. Numerical simulations have revealed coherent soliton configurations, also known as boson stars, in the centers of axion halos. The physical laws governing the interaction of boson stars with the halos are still largely unknown. I’ll report the results on evolution of axion solitons immersed into a gas of axion waves with Maxwellian velocity distribution obtained by combining analytical approach with controlled numerical simulations. It has been found that heavy solitons grow by condensation of axions from the gas, while light solitons evaporate. I’ll discuss the dependence of the soliton growth/evaporation rate on the soliton and gas parameters.

11/4/22: Presented by Dr. Yuhsin Tsai from the Perimeter Institute in Canada. The title of his talk is, “Anisotropy of the Stochastic Gravitational Wave Background from Primordial Fluctuations.”

Abstract: Gravitational waves produced in the primordial universe should show up as anisotropic stochastic gravitational wave backgrounds (GWB), similar to the cosmic microwave background (CMB). The GWB can carry the adiabatic perturbations as the CMB, or it can exhibit different energy fluctuations with non-minimal inflationary and reheating processes. In this talk, I will use GW produced by a first-order phase transition as an example to consider the anisotropy of the GWB that is either correlated or un-correlated to the CMB fluctuations. I will also explain the possibility of seeing a large non-Gaussianity (NG) signal in the GWB while obeying current observational bounds. Finally, I will discuss the prospects of distinguishing cosmological signals from the astrophysical foreground.

11/4/22: Presented by Dr. Xiaochuan Lu from the University of California, San Diego. The title of his talk is, “Standard Model Effective Field Theory and Beyond.”

Abstract: The Standard Model (SM) can be interpreted as a low-energy Effective Field Theory (EFT), by including non-renormalizable interactions that preserve SM symmetries. This well-known SMEFT framework provides a robust parameterization of the indirect impact of heavy new particles. In this talk, I will explain how to construct SMEFT operator basis with Hilbert series method, how to use SMEFT to perform practical calculations with functional methods, as well as why SMEFT is not always enough when there is non-decoupling physics beyond the SM.

10/21/22: Presented by Dr. Amara McCune from the University of California, Santa Barbara. The title of her talk is, “An Effective Cosmological Collider.”

Abstract: Particles with masses of order Hubble during inflation create a distinct, oscillatory signal in the squeezed limit of the CMB bispectrum. A host of models have been studied that produce non-Gaussianities in this limit, with a goal of identifying targets for near-future probes of the CMB and large-scale structure. Yet fully leveraging this program, known as "cosmological collider" physics, as a tool for particle discovery necessitates a systematic understanding of contributing operators and their effects. In this talk, we apply a rigorous effective field theory treatment to an Abelian Higgs model, which can represent either an SM toy model or a more general U(1) gauge theory in the early universe. We identify a minimal operator basis by enumerating all operators up to dimension 8, and classify their tree and one-loop behavior. In doing so, we eliminate all redundant operators and consider multiple operator effects, isolating the physical signatures of the theory. This leads to a signal for the Higgs that is projected to be accessible to near-future probes of large-scale structure, with particular reliance on 21cm probes.

10/14/22: Presented by Dr. Jedidiah Thompson from Stanford University in California. The title of his talk is, “Towards a Non-perturbative Construction of the S-matrix.”

Abstract: Despite many decades of progress in understanding quantum field theory, there are large gaps in our ability to compute at strong coupling. Recently, however, there has been significant progress in using Hamiltonian-based approaches formulated directly in Minkowski space, opening a new door to studying real-time dynamics. In particular, Hamiltonian truncation provides a concrete procedure for obtaining approximate energy eigenstates and eigenvalues for a QFT, and has been used successfully to compute detailed spectral information (particle masses, operator spectral densities, etc.) in nontrivial strongly-coupled QFTs. In this talk, I will explain how to use approximate knowledge of a QFT’s energy eigenstates to compute amplitudes for particle scattering. I will demonstrate this with an example of lightcone conformal truncation for the large-N O(N) model in 2+1d at strong coupling. Along with the amplitude in the physical region, it is possible to analytically continue to the entire complex Mandelstam plane, and in fact numerical convergence can be even better away from the physical region. Finally, I will comment on future directions and the conceptual steps still required to use these techniques for more complex theories such as QCD.

10/7/22: Presented by Dr. Zackaria Chacko from the University of Maryland. The title of his talk is, “Phenomenology of Partially Composite Neutrinos.”

Abstract: I will consider a class of models in which the neutrinos acquire their masses through mixing with singlet neutrinos that emerge as composite states of a strongly coupled hidden sector. In this framework, the light neutrinos are partially composite Majorana particles that obtain their masses through the inverse seesaw mechanism. I will focus on the scenario in which the strong dynamics is approximately conformal in the ultraviolet, and the compositeness scale lies at or below the weak scale. The small parameters in the Lagrangian necessary to realize the observed neutrino masses can naturally arise as a consequence of the scaling dimensions of operators in the conformal field theory. I will show that this class of models has interesting implications for a wide variety of experiments, including colliders and beam dumps, searches for lepton flavor violation and neutrinoless double beta decay, and cosmological observations.

9/30/22: Presented by Dr. Alexander Monin from University of South Carolina. The title of his talk is, “Semiclassics for U(1) charged CFT: A Brief Review.”

Abstract: It is generally expected that states with large quantum numbers can be described semiclassically. In the context of CFT with additional $U(1)$ symmetry these methods allow to find the spectrum of large charge primary operators in a class of models. I will explain the methodology with an example of a scalar theory at Wilson Fisher fixed point in $d=3-\epsilon$ dimensions and show how CFT data (scaling dimensions and fusion coefficients) can be obtained systematically as the inverse charge power series. I will also present a construction allowing to identify all spinning primary operators with number of derivatives bounded by the charge in a free 3d scalar theory.

9/16/22: Presented by Dr. Clifford Cheung from the California Institute of Technology (Caltech). The title of his talk is, “Double Copy Variations.”

Abstract: The double copy is an extraordinary structure relating the perturbative on-shell scattering amplitudes of gauge theory and gravity. In this talk, I describe progress towards a field theoretic understanding of the double copy from first principles, with an eye towards generalization to settings which are off-shell, beyond flat space, and non-perturbative. Focusing on scalar theories, I show how the double copy arises from a mapping of the color algebra to the diffeomorphism algebra. This permits an off-shell formulation of the double copy at the level of fields and equations of motion which immediately implies known amplitudes statements but also generalizes to correlators in curved geometries beyond AdS and dS.  In the simplified case of two spacetime dimensions, the double copy can be implemented non-perturbatively at the level of the action, and some of the relevant theories are integrable.  In this case I show how to double copy Wilson lines, charges, and non-perturbative field configurations.

9/9/22: Presented by Dr. Merab Gogberashvili from Javakhishvili State University & Andronikashvili Institute of Physics in Tbilisi, Georgia. The title of his talk is, “LIGO Signals from the Mirror World.”

Abstract: We suggest that a major fraction of binary black holes and neutron star mergers, which might provide gravitational wave signals detectable by LIGO/VIRGO, emerged from the hidden mirror sector. Mirror particles do not interact with an ordinary observer except gravitationally, which is the reason why no electromagnetic signals accompanying gravitational waves from mergers with components composed of mirror matter are expected. Mirror matter is a candidate of dark matter and its density can exceed ordinary matter density five times. Since the mirror world is considered to be colder, star formation there started earlier and mirror black holes had more time to pick up the mass and to create more binary systems within the LIGO reachable zone. Totally we estimate a factor 10 amplification of black holes merging rate in the mirror world with respect to our world, which is consistent with the LIGO observations. If the dark matter budget of the universe is mostly contributed by the mirror particles, we predict that only about one binary neutron star (neutron star - black hole) merger out of ten detectable by LIGO/VIRGO could be accompanied by a gamma ray burst. It seems the list of candidate events recorded by LIGO/VIRGO during the third observational run supports our predictions. We consider the possibility that LIGO events GW190521, GW190425 and GW190814 may have emerged from the mirror world binaries. Theories of star evolution predict so-called upper and lower mass gaps and masses of these merger components lie in those gaps. In order to explain these challenging events very specific assumptions are required and we argue that such scenarios are orders of magnitude more probable in the mirror world.

8/19/22: Presented by Dr. Yang Ma from the University of Pittsburgh in Pennsylvania. The title of his talk is, “The Partonic Picture and the SM Expectation of High-energy Lepton Colliders.”

Abstract: After the triumph of discovering the Higgs boson at the CERN Large Hadron Collider, people are getting increasingly interested in studying the Higgs properties in detail and searching for the physics beyond the Standard Model (SM). A multi-TeV lepton collider provides a clean experimental environment for both the Higgs precision measurements and the discovery of new particles. In high-energy leptonic collisions, the collinear splittings of the leptons and electroweak (EW) gauge bosons are the dominant phenomena, which could be well described by the parton picture. In the parton picture, all the SM particles should be treated as partons that radiated off the beam particles, and the electroweak parton distribution functions (EW PDFs) should be adopted as a proper description for partonic collisions of the initial states. Along this line, future ultra-high-energy lepton colliders can be treated as EW versions of the LHC. In our work, both the EW and the QCD sectors are included in the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) formalism to perturbatively resum the potential large logarithms emerging from the initial-state radiation (ISR). Using the EW PDF formalism, I will present the SM expectations of a possible high-energy electron-positron collider and a possible high-energy muon collider, which shall be included in the guideline of future analysis.

8/5/22: Presented by Dr. Joshua Eby from Tokyo University / IPMU in Japan. The title of his talk is, “Probing Ultralight Dark Matter and the Very Local Density from Earth and Space.”

Abstract: Ultralight dark matter (ULDM) is known to form self-gravitating bound states through gravitational relaxation. There are intriguing hints in the literature suggesting similar dynamics might lead to over densities in the solar system as well, with ULDM becoming bound to the Sun. These Solar Halos can be probed by experiments on Earth when their radius R is greater than 1 AU, which implies ULDM particle masses m is less than 10 to the {-14} eV. For larger masses m, space-based missions on orbits within 1 AU can probe small, compact Solar Halos with exceptional reach; for scalar couplings probable in current and near-future atomic clock systems, the sensitivity can exceed that of Equivalence Principle tests and probe well-motivated space for natural scalar field models. I will review the state of the art on these topics, including several exciting NASA and international missions that motivate searches aboard space probes.

5/13/22: Presented by Dr. Evgueni Goudzovski from the University of Birmingham in the United Kingdom. The title of his talk is, “Present and Future Kaon Experiments.”

Abstract: Rare kaon decays are among the most sensitive probes of both heavy and light new physics beyond the Standard Model description, thanks the high precision of the Standard Model predictions, the availability of very large datasets, and the relatively simple decay topologies. The NA62 experiment at CERN has reported the first observation of the ultra-rare K+ to pi+nunu decay, and is collecting data towards a 10% measurement of the decay rate. A plan for a comprehensive program to study K+ and KL rare decays at CERN beyond NA62 is currently taking shape. The KOTO experiment at J-PARC is approaching the SM sensitivity to the ultra-rare KL to pi0nunu decay, and the next step of the KOTO program has been proposed. Both NA62 and KOTO experiments pursue broad rare-decay and hidden-sector physics programs. Recent results and future plans for kaon experiments are discussed.

4/29/22: Presented by Dr. Timothy Cohen from the University of Oregon. The title of his talk is, “A Tale of Eternal Inflation.”

Abstract: In this talk, I will present a new result where we incorporate the effects of primordial non-Gaussianity into the framework of Stochastic Inflation for the scalar fluctuations of the inflaton. I will then show that applying this result to the parameter space that is being probed by observations leads to a surprising conclusion that can be interpreted as a breakdown of effective field theory in this regime. The calculations are performed using the framework of Soft de Sitter Effective Theory, and I will review this formalism and will show how it can be applied to the simpler setting of massless scalar fields in a fixed dS background, where no apparent breakdown of the EFT is observed.

4/22/22: Presented by Dr. Wei Xue from the University of Florida. The title of his talk is, “Exploring New Physics from the Laboratory to the Early Universe.”

This talk is only available by request for University of Minnesota faculty, staff, and students.  
If you would like to watch this talk, please email Alicia Canfield for a link. 

4/8/22: Presented by Dr. John Terning from the University of California, Davis. The title of his talk is, “Scattering Amplitudes for Monopoles and the Pairwise Little Group.”

Abstract: After a brief review of the challenges of calculating scattering amplitudes for electric and magnetic charges, as well as a discussion of Wigner’s “little group” and the helicity amplitude approach, I will discuss how S-matrix calculations can be extended to include magnetic charges. The answer involves an extension of helicity to a "pairwisehelicity" involving two particles. With this in hand, the notorious Rubakov-Callan processes turn out to be quite simple.

4/1/22: Presented by Dr. Saarik Kalia from Stanford University in California. The title of his talk is, “Earth as a Transducer for Ultralight Dark-matter Detection.”

Abstract: In this talk, I will propose the use of the Earth as a transducer for ultralight dark-matter detection. In particular I will point out a novel signal of both kinetically mixed dark-photon dark matter and axion like dark matter: a monochromatic oscillating magnetic field generated at the surface of the Earth. Similar to the signal in a laboratory experiment in a shielded box (or cavity), this signal arises because the lower atmosphere is a low-conductivity air gap sandwiched between the highly conductive interior of the Earth below and ionosphere or interplanetary medium above. For dark-photon dark matter, the kinetic mixing with the Standard Model photon allows dark matter to convert into an observable magnetic field inside this cavity, while for axion dark matter, the background geomagnetic field of the Earth allows the axion to convert through its coupling to photons. The magnetic field signal of ultralight dark matter in a laboratory detector is usually suppressed by the size of the detector. Crucially, in our case the suppression is by the radius of the Earth, and not by the (much smaller) height of the atmosphere. The magnetic field signal exhibits a global vectorial pattern that is spatially coherent across the Earth, which enables sensitive searches for this signal using unshielded magnetometers dispersed over the surface of the Earth. I will summarize the results of such a search using a publicly available dataset from the SuperMAG collaboration. The dark-photon dark matter constraints from this search are complementary to existing astrophysical bounds, and the axion dark matter constraints are comparable to the bounds obtained by the CAST helioscope. Future searches for this signal may improve the sensitivity over a wide range of masses for both ultralight dark-matter candidates.

3/18/22: Presented by Dr. Gonzalo Alonso-Alvarez from McGill University in Canada. The title of his talk is, “The Strange Physics of Dark Baryons.”

Abstract: The origin of dark matter and the matter-antimatter asymmetry of the Universe may be explained by the existence of GeV-scale dark sector particles carrying baryon number. The interactions of such dark baryons with first-generation quarks are known to have implications for collider experiments, neutron stars, and the lifetime of the neutron. After reviewing these topics, I will focus on the phenomenology of dark baryon interactions with strange quarks. This includes their impact on the decay of exotic hadrons, core-collapse supernova explosions, and new physics searches at the LHC and flavour physics experiments.

3/4/22: Presented by Dr. Inar Timiryasov from the Niels Bohr Institute in Denmark. The title of his talk is, “Low-scale Leptogenesis.”

Abstract: Right-handed neutrinos offer an elegant solution to two well-established phenomena beyond the Standard Model (SM)—masses and oscillations of neutrinos, as well as the baryon asymmetry of the Universe. It is also a minimalistic solution since it requires only singlet Majorana fermions to be added to the SM particle content. If these fermions are nearly degenerate, the mass scale of right-handed neutrinos can be very low and accessible by the present and planned experiments. There are at least two well-studied mechanisms of low-scale leptogenesis: baryogenesis via oscillations and resonant leptogenesis. These two mechanisms were often considered separate, but they can, in fact, be understood as two different regimes of one and the same mechanism, described by a unique set of quantum kinetic equations. In this work, we show, using a unified description based on quantum kinetic equations, that the parameter spaces of these two regimes of low-scale leptogenesis significantly overlap. We present a comprehensive study of the parameter space of low-scale leptogenesis with the mass scale ranging from 0.1 to ∼10^6  GeV. The unified perspective of our works reveals the synergy between intensity and energy frontiers in the quest for heavy Majorana neutrinos.

2/25/22: Presented by Dr. Francesco D'Eramo from the University of Padova in Italy. The title of his talk is, “Cosmological Imprints of Hot Axions.”

Abstract: Scattering and decay processes of thermal bath particles in the early universe can dump hot axions in the primordial plasma, and they would manifest themselves in the cosmic microwave spectrum as additional neutrinos. In this talk, I will review predictions for such an effect due to different axion couplings. Finally, I will present predictions for concrete UV complete axion models and explore the discovery reach of future cosmological surveys.

2/18/22: Presented by Dr. Matthew Mccullough from CERN in Switzerland. The title of his talk is, “Self-organized Localisation.”

Abstract:  I will describe a new phenomenon in quantum cosmology: self-organised localisation. When the fundamental parameters of a theory are functions of a scalar field subject to large fluctuations during inflation, quantum phase transitions can act as dynamical attractors. As a result, the theory parameters are probabilistically localised around the critical value and the Universe finds itself at the edge of a phase transition. We illustrate how self-organised localisation could account for the observed near-criticality of the Higgs self-coupling, the naturalness of the Higgs mass, or the smallness of the cosmological constant.

2/11/22: Presented by Dr. JiJi Fan from Brown University in Rhode Island. The title of her talk is, “Axion Echos from Supernovae Remnants.”

Abstract: Stimulated decays of axion dark matter, triggered by a source in the sky, could produce a photon flux along the continuation of the line of sight, pointing backward to the source. The strength of this so-called axion “echo” signal depends on the entire history of the source and could still be strong from sources that are dim today but had a large flux density in the past, such as supernova remnants (SNRs). This echo signal turns out to be most observable in the radio band. I will present the sensitivity of radio telescopes such as the Square Kilometer Array (SKA) to echo signals generated by SNRs that have already been observed. In addition, I will show projections of the detection reach for signals from newly born supernovae that could be detected in the future. Intriguingly, an observable echo signal could come from old “ghost” SNRs which were very bright in the past but are now so dim that they haven’t been observed.

2/4/22: Presented by Dr. Michael Zantedeschi from the Max Planck Institute in Munich, Germany. The title of his talk is, “Vortexes in Black Holes.”

Abstract: In this talk I will argue that black holes admit vortex structure. This is based both on a graviton-condensate description of a black hole as well as on a correspondence between black holes and generic objects with maximal entropy compatible with unitarity, so-called saturons. Due to vorticity, a Q-ball-type saturon of a calculable renormalisable theory obeys the same extremality bound on the spin as the black hole. Correspondingly, a black hole with extremal spin emerges as a graviton condensate with vorticity. Next, I will show that in the presence of mobile charges, the global vortex traps a magnetic flux of the gauge field. This can have macroscopically-observable consequences. For instance, the most powerful jets observed in active galactic nuclei can potentially be accounted for. As a signature, such emissions can occur even without a magnetized accretion disk surrounding the black hole. The flux entrapment can provide an observational window to various hidden sectors, such as millicharged dark matter.

1/28/22: Presented by Dr. Jessie Shelton from the University of Illinois at Urbana-Champaign. The title of her talk is, “Nonstandard Thermal Histories and the Small-scale Matter Power Spectrum.”

Abstract: Decoupled hidden sectors in the early universe can easily and generically result in departures from radiation domination prior to BBN, leaving a potentially observable footprint in the distribution of dark matter on very small scales. I'll talk about the gravitational consequences of an era of modified cosmic expansion, maps to the particle physics of decoupled hidden sectors that can realize such eras, and the features in the (linear) matter power spectrum, with an eye toward observability.

1/21/22: Presented by Dr. Lingfeng Li from Brown University in Rhode Island. The title of his talk is, “Varying Higgs VEV in Cosmology and an Axionic Solution.”

Abstract:  The Lambda CDM model provides an excellent fit to the CMB data. However, a statistically significant tension emerges when its determination of the Hubble constant H 0 is compared to the local distance-redshift measurements. The axi-Higgs model, which couples an ultralight axion to the Higgs field, offers a specific variation of the  Lambda CDM model. It relaxes the H 0 tension as well as explains the 7 Li puzzle in Big-Bang nucleosynthesis, the clustering S 8 tension with the weak-lensing data, and the observed isotropic cosmic birefringence in CMB.

12/10/2021: Presented by Dr. Mohamed Anber from Durham University in the United Kingdom. The title of his talk is, “The Global Structure of the Standard Model and New Non-perturbative Processes.”

Abstract: It is well-established that the Standard Model (SM) of particle physics is based on SU(3) X SU(2) X U(1) Lie-algebra. What is less appreciated, however, is that SM accommodates a Z_6 1-form global symmetry.  Gauging this symmetry, or a subgroup of it, changes the global structure of the SM gauge group and amounts to summing over sectors of instantons with fractional topological charges. After a brief review of the concept of higher-form symmetries, I will explain the origin of the Z_6 1-form symmetry and construct the explicit fractional-instanton solutions on compact manifolds. The new instantons mediate baryon-number and lepton-number violating processes, which can win over the weak BPST-instanton processes, provided that SM accommodates extra hyper-charged particles above the TeV scale. I will also comment on the cosmological aspects of the new solutions.

12/3/2021: Presented by Dr. Csaba Csaki from Cornell University in New York. The title of his talk is, “Exploring the phases of gauge theories via Anomaly Mediated Supersymmetry Breaking (AMSB).”

Abstract: Finding the vacuum structure of strongly coupled gauge theories is one of the important unsolved questions in particle physics. Within supersymmetric (SUSY) theories many of these questions have been largely resolved in the 1990's following the work of Seiberg and others, however so far we have not been able to convincingly connect these results to their non-supersymmetric counterparts. Recently Murayama proposed to use anomaly mediated supersymmetry breaking (AMSB) to introduce the SUSY breaking terms which allows finding results consistent with the qualitative expectations for the structure of the non-SUSY theories. In this talk I first show how to apply this method to a class of chiral gauge theories based on antisymmetric and symmetric representations, which leads us to propose novel symmetry breaking patterns for the vacuum of these theories, and calls for modification of the old tumbling picture of confinement in chiral gauge theories. I then apply the method to the SO(N) series and show that for F < 3/2 (N-2) the theory will be confining, where the dynamics of confinement is monopole condensation, and identify the resulting global symmetry breaking pattern.

11/19/2021: Presented by Dr. Brian Batell from the University of Pittsburgh in Pennsylvania. The title of his talk is, “Thermal Misalignment of Scalar Dark Matter.”

Abstract: The conventional misalignment mechanism for scalar dark matter depends on the initial field value, which governs the oscillation amplitude and present-day abundance. We present a mechanism by which a feeble (Planck-suppressed) coupling of dark matter to a fermion in thermal equilibrium drives the scalar towards its high-temperature potential minimum at large field values, dynamically generating misalignment before oscillations begin. Unlike conventional misalignment production, the dark matter abundance is dictated by microphysics and not by initial conditions. As an application of the generic mechanism, we discuss a realistic scenario in which dark matter couples to the muon.

11/12/2021: Presented by Dr. Michael Fedderke from Johns Hopkins University in Maryland. The title of his talk is, “Bridging the Microhertz Gap with Asteroids: Opportunities and Challenges for Gravitational Wave Detection.”

Abstract: The science case for a broad program of gravitational wave (GW) detection across all frequency bands is exceptionally strong. At present, there is a dearth of coverage by existing and proposed searches in the GW frequency band lying between the peak sensitivities of PTAs and LISA, roughly 0.1-100 microhertz. In this talk, I will outline a conceptual mission proposal to access this band. I will demonstrate that a few carefully chosen asteroids which orbit in the inner Solar System can act as excellent naturally occurring gravitational test masses despite the environmental noise sources. As such, a GW detector can be constructed by ranging between these asteroids using optical or radio links. At low frequencies, I will discuss how gravity gradient noise arising from the combined motion of the other ~million asteroids in the inner Solar System sharply cuts off the sensitivity of this proposal. Sensitivity in the middle of this band is mostly limited by various solar perturbations to the asteroid test masses, while the high-frequency sensitivity is limited by noise in the ranging link. The projected strain-sensitivity curve that I will present indicates significant potential reach in this frequency band for a mission of this type.

11/5/2021: Presented by Dr. Yang Bai from the University of Wisconsin, Madison. The title of his talk is, “Q-monopole-ball.”

Abstract: A soliton state with both topological and non-topological charges is shown to exist in a certain broken gauge theory with an additional global U(1) symmetry. This new soliton state, Q-monopole-ball, is shown to be stable from decaying into an isolated Q-ball and a magnetic monopole state. There exists a minimum global charge for a Q-monopole-ball being stable from decaying into free global U(1)-charged particles and an isolated monopole. Q-monopole-balls with a large magnetic charge can also be stable if the topological charge is smaller than the cubic root of the global charge. Q-monopole-balls could be produced from a phase transition in the early universe and account for all dark matter.

10/29/2021: Presented by Dr. Andrew Long from Rice University in Texas. The title of his talk is, “Searching for New Physics with X-rays from Compact Stars.”

Abstract: Since axions couple extremely weakly to regular matter, it makes them challenging to probe in the laboratory. However, axions should be produced in the dense environments of compact stars. Stellar axion emission provides an additional cooling channel that leads to well-known constraints on the axion’s couplings to matter. These constraints are indirect, and although compact stars are predicted to “glow” in axions, this radiation is invisible to us. In this talk I will discuss how the axion radiation is converted into X-ray emission in the strong magnetic field that surrounds many compact stars, thereby providing a new strategy for probing axions through X-ray observations of white dwarfs and neutron stars.

10/22/2021: Presented by Dr. Liam Fitzpatrick from Boston University in Massachusetts. The title of his talk is, “Extracting Dynamics in Quantum Field Theory from Conformal Field Theory Data.”

Abstract: A compelling view of Quantum Field Theories (QFTs) is that they are points along the RG flow between fixed points described by Conformal Field Theories (CFTs), which in turn are fully characterized by a discrete set of "CFT data". In this talk, we describe how this picture can be turned into a useful calculational tool for studying QFT at strong coupling by applying a variational method motivated by the conformal structure of the ultraviolet CFT fixed point of the theory. We focus on two applications, 2d phi^4 theory near its critical point, and 2d QCD with three colors and one fundamental quark in the chiral limit where the quark mass is small but nonzero.

10/15/2021: Presented by Dr. Rodolfo Capdevilla from the Perimeter Institute in Canada. The title of his talk is, “Discovering the new physics of (g−2)μ at colliders.”

Abstract: The Fermilab Muon g−2 collaboration has recently released its first measurement of (g−2)μ. This result is consistent with previous Brookhaven measurements and together they yield a statistically significant 4.2σ discrepancy with the Standard Model prediction. BSM solutions to (g−2)μ feature light weakly coupled neutral particles (Singlet Scenarios) or heavy strongly coupled charged particles (Electroweak Scenarios). In recent investigations, it has been shown how a 3TeV muon collider (MuC) can probe all possible Singlet Scenarios, whereas a 30TeV MuC is guaranteed to produce the heavy states in the Electroweak Scenarios under a set of reasonable assumptions. In this talk I will summarize these findings and present new developments. On one hand, a combination of hadron colliders and precision electroweak measurements can probe an important portion of the parameter space in the Singlet Scenarios. This is for heavy singlets in the range between 10 GeV and 1-3 TeV. On the other hand, Electroweak Scenarios where BSM states are too heavy to be produced at any foreseen collider can still be probed by indirect signatures at a MuC. One example in the literature is Higgs+gamma production at a 30TeV MuC. Here, we probe the heaviest Electroweak Scenarios for (g−2)μ looking at di-Higgs production at a 10TeV MuC.

10/8/2021: Presented by Dr. Mikhail Goykhman from the Fine Theoretical Physics Institute at the University of Minnesota. The title of his talk is, “Long-range Vector Models and Critical Dualities.”

Abstract: I will discuss some recent developments in the large-N long-range critical vector models. In particular, I will focus on the generalized free O(N) scalar field deformed by quartic interaction. For certain choice of parameters, quartic interaction drives this model to a non-trivial critical regime in the UV. I will show that the same critical regime exists at the IR end of an RG flow of a different long-range model, thereby furnishing a non-trivial example of the critical duality of long-range models. I will also comment on the fermionic generalization of this construction.

10/1/2021: Presented by Dr. Anna Tokareva from Jyvaskyla University in Finland. The title of her talk is, “Four-dimensional Treatment of Positivity Bounds with Gravity.”

Abstract: We formulate Positivity Bounds for scattering amplitudes including exchange of gravitons in four dimensions. We generalize the standard construction through dispersion relations to include the presence of a branch cut along the real axis in the complex plane for the Mandelstam variable s. In general, validity of these bounds require the cancellation of divergences in the forward limit of the amplitude. We show that this is possible only if one assumes a Regge behavior of the amplitude at high energies. As a non-trivial fact, a concrete UV behavior of the amplitude is uniquely determined by the structure of IR divergences. We discuss also possible phenomenological applications of these bounds.

9/24/2021: Presented by Dr. Surjeet Rajendran from Johns Hopkins University in Maryland. The title of his talk is, "A Causal Framework for Non-Linear Quantum Mechanics."

Abstract: We add non-linear and state-dependent terms to quantum field theory. We show that the resulting low-energy theory, non-linear quantum mechanics, is causal, preserves probability and permits a consistent description of the process of measurement. We explore the consequences of such terms and show that non-linear quantum effects can be observed in macroscopic systems even in the presence of de-coherence. We find that current experimental bounds on these non-linearities are weak and propose several experimental methods to significantly probe these effects. We also expose a fundamental vulnerability of any non-linear modification of quantum mechanics - these modifications are highly sensitive to cosmic history and their locally exploitable effects can dynamically disappear if the observed universe has a tiny overlap with the overall quantum state of the universe, as is predicted in conventional inflationary cosmology. We identify observables that persist in this case and discuss opportunities to detect them in cosmic ray experiments, tests of strong field general relativity and current probes of the equation of state of the universe. Non-linear quantum mechanics also enables novel gravitational phenomena and may open new directions to solve the black hole information problem and uncover the theory underlying quantum field theory and gravitation.

9/17/2021: Presented by Dr. Ilaria Brivio from the Institute of Theoretical Physics and the University of Heidelberg in Germany. The title of her talk is "The Neutrino Option.”

Abstract: The core idea of the Neutrino Option is that the Higgs potential of the SM could be naturally generated by loops of right-handed neutrinos, starting from a nearly-conformal condition in a very minimal type-I seesaw model. After introducing the generalities of this framework, I will discuss its compatibility with leptogenesis and I will give an overview of the plausible UV completions.

9/10/2021: Presented by Dr. Jack Collins from SLAC National Accelerator Laboratory in California. The title of his talk is, “The Learnt Geometry of Collider Events.”

Abstract: Collider events, when imbued with a metric which characterizes the 'distance' between two events, can be thought of as populating a data manifold in a metric space. The geometric properties of this manifold reflect the physics encoded in the distance metric. I will show how the geometry of collider events can be probed using a class of machine learning architectures called Variational Autoencoders.

4/30/2021: Presented by Dr. Mustafa Amin from Rice University in Texas. The title of his talk is, “Light from Dark Solitons.”

Abstract: Axions and axion-like fields are popular in cosmology, both as the inflaton and as dark matter. Such fields can naturally condense into long-lived, spatially localized configurations (oscillons, axion stars etc). I will discuss conditions under which such solitons become effective antennas for electromagnetic radiation by converting axions to photons, which can lead to potential observational signatures. I will also discuss multimessenger signals: electromagnetic and gravitational wave emission from soliton collisions.  Time permitting, I will digress to advertise a separate topic of CMB birefringence from a different type of soliton: string loops in ultralight-axions.

4/16/2021: Presented by Dr. Raffaele-Tito D'agnolo from Universite Paris-Saclay in France. The title of his talk is, “The Weak Scale as a Trigger.”

Abstract: I will discuss settings where the Higgs mass squared affects the vacuum expectation value of local operators and can thus act as a “trigger” of new cosmological dynamics. This triggering mechanism underlies several existing solutions to the hierarchy problem that trace the origin of the weak scale to the early history of the Universe. Thinking about these solutions more systematically from the point of view of weak scale triggers allows us to understand their common predictions, to find new solutions and to identify unexpected physics related to naturalness in a rather model-independent way. As an example I discuss a BSM trigger in a Two Higgs Doublet Model and show how it can be used to link the tuning of the Higgs mass to that of the cosmological constant. This weak scale trigger demands the existence of new Higgs states necessarily comparable to or lighter than the weak scale, with no wiggle room to decouple them.

4/2/2021: Presented by Dr. Vedran Brdar from Northwestern University in Illinois. The title of his talk is, “Gravitational Waves as a Probe of New Physics: From LIGO to NANOgrav.”

Abstract: In the first part of the talk I will discuss gravitational wave signature arising from first order phase transition in two different models featuring neutrino mass generation through type-I seesaw mechanism. The expected gravitational wave spectra from these models will be confronted with sensitivities of ground-based detectors such as LIGO as well as several future space-based observatories. I will show that in case current and future gravitational wave observatories find stochastic gravitational wave component that is not of astrophysical origin, such beyond the Standard Model signature would hint scale-invariant dynamics. In the second part of the talk I will discuss recent work on the gravitational wave production from topological defects. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has recently reported strong evidence for a stochastic common-spectrum process affecting the pulsar timing residuals in its 12.5-year data set. I will show that this process admits an interpretation in terms of a stochastic gravitational-wave background emitted by a cosmic-string network in the early Universe.

3/26/2021: Presented by Dr. Haipeng An from Tsinghua University in China. The title of his talk is, “Gravitational Waves from First-order Phase Transition During Inflation.”

Abstract: During the inflation era, the properties (such as mass and interactions) of the fields coupled to the inflaton field may change substantially. As a result, drastic phenomena, such as first order phase transitions, may happen. In this talk, I will present simple models that first-order phase transition can happen and finish during inflation. I will discuss the properties of the gravitational wave (GW) signals produced by first-order phase transitions during inflation. I will show that there is a unique oscillatory feature in the GW spectrum. I will also show that we may be able to observe directly such a signal through future terrestrial or spatial GW detectors.

3/19/2021: Presented by Dr. Daniel Carney from the University of Maryland and LBNL/NIST. The title of his talk is, “Quantum limits, Mechanical sensing, and Dark matter.”

Abstract: Nearly forty years ago, Caves, Thorne, and their collaborators asked: what are the quantum-mechanical limits to the detection of a small change in the position of an object? Limits of this type are becoming ubiquitous in modern quantum-limited detectors, and eventually will need to be confronted in a wide variety of low-threshold detection problems. I'll briefly review the basic theory of quantum measurement noise, and present applications in the search for dark matter. In particular, mechanical detectors--the LIGO pendula being a canonical example--appear poised to make substantial contributions. After discussing some current and near-future experimental work, I'll present a concept for an optomechanical detection scheme which, near the limits of what is possible according to quantum mechanics, would be capable of direct detection of sufficiently heavy dark matter candidates purely through their gravitational interactions with the device.

3/12/2021: Presented by Dr. Christopher Verhaaren from the University of California, Irvine, the title of his talk is, “Learning from Dark Monopoles.”

Abstract: Magnetic monopoles appear in many motivated quantum field theories, and are intimately tied to electric charge quantization. This same quantization condition implies that the monopoles of our familiar electrodynamics have a large, nonperturbative, coupling to the photon, making it difficult to provide trustworthy theoretical and phenomenological predictions regarding the interaction between electric and magnetic particles. If, however, a dark sector with magnetic monopoles of a dark U(1) gauge field has a small mixing with our photon, then the dark monopoles can develop a small, perturbative, magnetic coupling to our photon. This provides a theoretical laboratory for learning about the interactions between electric and magnetic charges in quantum field theory. I outline how to use the Zwanziger Lagrangian to understand kinetic mixing in the context of electric and magnetic charges. I explain how we can understand perturbative interactions between electric and magnetic particles, and outline aspects of the phenomenology of dark monopoles.

3/5/2021: Presented by Dr. Irene Valenzuela from Harvard University in Massachusetts, the title of her talk is, “Chern-Weil Global Symmetries and How Quantum Gravity Avoids Them.”

Abstract: I will discuss a class of generalized global symmetries, which we call “Chern-Weil global symmetries,” that arise ubiquitously in gauge theories. The Noether currents of these Chern-Weil global symmetries are given by wedge products of gauge field strengths and their conservation follows from Bianchi identities, so they are not easy to break. However, exact global symmetries should not be allowed in a consistent theory of quantum gravity. I will explain how these symmetries are typically gauged or broken in string theory. Interestingly, many familiar phenomena in string theory, such as axions, Chern-Simons terms, world-volume degrees of freedom, and branes ending on or dissolving in other branes, can be interpreted as consequences of the absence of Chern-Weil symmetries in quantum gravity, suggesting that they might be general features of quantum gravity.

2/26/2021: Presented by Dr. Anson Hook from the University of Maryland, the title of his talk is, “Fun with Cosmic Strings: A CMB Millikan Experiment and Colliders in the Sky.”

Abstract: We discuss two unique signatures of cosmic strings.  Photons moving around an axion string undergo an Aharonov-Bohm effect.  As a result, axion strings produce a distinct quantized polarization rotation of CMB photons which can be as large as O(1%).  The quantized polarization rotation angle is topological in nature and its value provides insight into the quantization of electric charge.  On the flip side, axion strings moving through electric and magnetic fields obtain extremely large currents.  When these currents collide, they shine with the brightness of 10^7 suns giving a unique signature that is observable at current and future telescopes.

2/19/2021: Presented by Dr. Manuel Buen-Abad from Brown University in Rhode Island, the title of his talk is, “Constraints on Axions from Cosmic Distance Measurements.”

Abstract: Axion couplings to photons could induce photon-axion conversion in the presence of magnetic fields in the Universe. The conversion could impact various cosmic distance measurements such as luminosity distances to type Ia supernovae and angular distances to galaxy clusters in different ways. We consider different combinations of the most updated distance measurements to constrain the axion-photon coupling. Ignoring the conversion in intracluster medium (ICM), we find the upper bounds on axion-photon couplings to be around 5 × 10^−12 (nG/B) GeV^−1 for axion mass below 5 × 10^−13 eV, where B is the strength of the magnetic field in the intergalactic medium (IGM). When including the conversion in ICM, the upper bound gets stronger and could be as strong as 5 × 10^−13 GeV^−1 for m a < 5 × 10^−12 eV. While this stronger bound depends on the ICM modeling moderately, it is independent of the IGM parameters.

2/12/2021: Presented by Dr. Alexander Monin from Lausanne University in Switzerland, the title of his talk is, “Multiparticle Processes, Large Charge and Semiclassics.”

Abstract: Understanding the behavior of a cross section at high enough energies, when the number of particles in the final state is large, is an important and yet unsolved subject. The difficulty in addressing the question is in-applicability of the standard perturbation theory for describing processes with many quanta even in weakly coupled theories. However, a certain reorganization (similar to RG improvement) of the perturbative expansion indicates a possibility of a semi-classical description: in other words perturbation theory around a non-trivial saddle. First, I’ll present the current state of affairs with regards to a scalar $lambda \phi^4$ theory. And later I’ll show an explicit and consistent construction allowing to compute observables like anomalous dimension etc. for operators with arbitrary large charge in $U(1)$ symmetric scalar field theory at Wilson-Fisher fixed point.

2/5/2021: Presented by Dr. Nicholas Orlofsky from Carleton University in Canada, the title of his talk is, “Electroweak-symmetric Relics.”

Abstract: Massive and extended relics are interesting dark matter candidates. If they have non-trivial interactions with the Standard Model electroweak sector, the electroweak vacuum can be modified inside or around such relics. In the most striking cases, electroweak symmetry is restored within a fixed macroscopic radius. I will discuss how this can happen in both the Standard Model and beyond the Standard Model examples. I will also discuss the phenomenological consequences and search strategies.

1/29/2021: Presented by Dr. Erich Poppitz from the University of Toronto in Canada, the title of his talk is, “Confinement on R3 x S1 and Double-String Collapse.”

Abstract: Confining strings in supersymmetric Yang-Mills theory with one spatial dimension compactified to a circle have been conjectured to consist of two domain walls arranged into a “double string” and carrying the chromoelectric flux of static quark sources. After explaining the setup, I will show that the double-string confinement mechanism holds for quarks of all N-alities, except for fundamental quarks, for which the domain walls collapse to form a single flux tube. I will also discuss the unusual N-ality dependence of the string tensions and their behaviour upon increasing the circle size.

1/22/2021: Presented by Dr. Quentin Bonnefoy from the German Electron Synchrotron (DESY) in Germany, the title of his talk is, “EFTs and Anomalies Revisited: SMEFT Sum-rules and Axion Couplings.”

Abstract: I will discuss the two following questions: (i) are there new anomaly cancellation conditions in the standard model (SM) effective field theory (SMEFT) beyond those of the SM? (ii) which number enters the scattering amplitude of an axion and two gauge bosons? Regarding (i), I will explain why the coefficients of SMEFT gauge-invariant operators which modify fermion gauge couplings can be chosen at will. Regarding (ii), I will present how models of massive chiral gauge fields evade the usual answer, according to which the number is a UV anomaly coefficient.

1/15/2021: Presented by Dr. Andrey Shkerin from the William I. Fine Theoretical Physics Institute at the University of Minnesota, the title of his talk is, “Inflation and Dark Matter Production in Einstein-Cartan Gravity.”

Abstract: We will discuss gravity coupled non-minimally to scalar and fermion fields in the Einstein-Cartan framework. We will focus on two phenomenological implications of Einstein-Cartan gravity. First, identifying a scalar with the Higgs field leads to inflation which generalizes the models of Higgs inflation in the metric and Palatini formulations of gravity and which is consistent with observations for a broad range of parameters. Second, including singlet fermions into consideration leads to the gravitational mechanism of their production in the Early Universe. We will show that fermions produced this way can constitute dark matter in a broad range of fermion masses: from a few keV up to 10^8 GeV.

12/11/2020: Presented by Dr. Marco Hufnagel from the German Electron Synchrotron (DESY) in Germany, the title of his talk is, "Updated BBN Constraints on Electromagnetic Decays of MeV-scale Particles."

Abstract: In this work, we revise and update model-independent constraints from Big Bang Nucleosynthesis on MeV-scale particles Φ which decay into photons and/or electron-positron pairs. We use the latest determinations of primordial abundances and extend the analysis from 1808.09324 by including all spin-statistical factors as well as inverse decays, significantly strengthening the resulting bounds in particular for small masses. For a very suppressed initial abundance of Φ, these effects become ever more important and we find that even a pure 'freeze-in' abundance can be significantly constrained. Besides, we also present our new public code ACROPOLIS, which numerically solves the reaction network necessary to evaluate the effect of photo disintegration on the final light element abundances. Including the process of photo disintegration into the numerical analysis is e.g. especially important for the scenarios discussed in this work, as it can significantly change the abundances after standard BBN due to late-time high-energy injections, thus leading to more stringent limits.

12/4/2020: Presented by Dr. Patrick Draper from the University of Illinois at Urbana-Champaign, the title of his talk is, "de Sitter Decays to Infinity."

Abstract: The bubble of nothing is a gravitational instanton that can be thought of as describing tunneling through a vanishing scalar potential barrier to a vacuum at infinity. I discuss generalizations of this process to nonzero scalar potentials with metastable de Sitter vacua. In simple cases, approximate bubble-of-nothing solutions can be constructed analytically. More generally, the problem can be formulated as a set of CdL equations with singular boundary conditions and solved numerically.

11/20/2020: Presented by Dr. Yi-Ming Zhong from the Kavli Institute for Cosmological Physics at the University of Chicago in Illinois, the title of his talk is, “A New Mask for an Old Suspect.”

Abstract: The Fermi-LAT Collaboration has released a new point source catalog, referred to as 4FGL. For the first time, we perform a template fit using information from this new catalog and find that the Galactic center excess is still present. On the other hand, we find that a wavelet-based search for point sources is highly sensitive to the use of the 4FGL catalog: no excess of bright regions on small angular scales is apparent when we mask out 4FGL point sources. We postulate that the 4FGL catalog contains the large majority of bright point sources that have previously been suggested to account for the excess in gamma rays detected at the Galactic center in Fermi-LAT data. Furthermore, after identifying which bright sources have no known counterpart, we place constraints on the luminosity function necessary for point sources to explain the smooth emission seen in the template fit. Further details can be found in arXiv: 1911.12369.

11/13/2020: Presented by Dr. Nicholas Rodd from the University of California, Berkeley, the title of his talk is, “Spectra for Heavy Dark Matter.”

Abstract: In this talk I will outline how to compute the decay spectrum for dark matter with masses above the scale of electroweak symmetry breaking, all the way to the Planck scale. These spectra are a crucial ingredient in the search for dark matter via indirect detection at the highest energies as being probed in current and upcoming experiments including IceCube, HAWC, CTA, and LHAASO. The method described improves considerably on existing approaches. For example, I will outline how to include all relevant electroweak interactions. The importance of these effects grow with dark matter mass, and by an EeV the spectra can differ by orders of magnitude from existing results.

10/30/2020: Presented by Dr. Kevin Kelly from the Fermi National Accelerator Laboratory in Illinois, the title of his talk is, “Heavy Neutrinos and Where to Find Them.”

Abstract:  The discovery of neutrino oscillations led to a new understanding that neutrinos have mass, which requires physics beyond the Standard Model. One well-motivated and well-studied solution is that right handed neutrinos exist and interact in a way that generates light neutrino masses. Moreover, if these new neutrinos are “Heavy”, there is potential for explaining why the Standard Model neutrinos are so much lighter than the charged leptons and quarks. I will summarize current searches for these heavy neutrinos across a wide range of masses and then focus on a particular regime of interest — GeV-scale Heavy Neutrinos. I will demonstrate how neutrino oscillation experiments can serve as a great environment to find these hypothetical particles in the coming decade. If we are lucky enough to discover these particles, then understanding them will become of paramount importance to the particle physics community. I will show strategies for exploring two specific characteristics of these heavy neutrinos by studying their decays: whether or not Lepton number is conserved (or whether they are Dirac or Majorana fermions), and what types of particle/particles mediate their interactions.

10/23/2020: Presented by Dr. Avner Karasik from Cambridge University in the United Kingdom, the title of his talk is, “Vector Dominance, One Flavored Baryons, and QCD Domain Walls.”

Abstract: Recently it has been proposed that the vector mesons in QCD have a special role as Chern-Simons fields on various QCD objects such as domain walls and one flavored baryons. I will argue that this proposal coincides with the conditions for vector meson dominance in the large N limit. This observation provides an experimental evidence for this proposal as well as a theoretical explanation to vector dominance. I will also discuss applications to Seiberg duality between gluons and vector mesons and mention some new results regarding QCD domain walls.

10/16/2020: Presented by Dr. Jure Zupan at the University of Cincinnati in Ohio, the title of his talk is, “Flavor Violating Axions.”

Abstract: Axion models with generation-dependent Peccei-Quinn charges can lead to flavor-changing neutral currents, thus motivating QCD axion searches at precision flavor experiments. I will cover the constraints on both quark flavor violating decays into axions as well as leptonic ones, both at precision laboratory experiments as well as in astrophysics. I will present a proposal for a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target.  I will discuss the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle.

10/09/2020: Presented by Dr. Alexey Milekhin from Princeton University in New Jersey, the title of his talk is, “Quantum Error Correction and Large N.”

Abstract: In recent years quantum error correction (QEC) has become an important part of AdS/CFT. Unfortunately, there are no field-theoretic arguments about why QEC holds in known holographic systems. The purpose of this talk is to fill this gap by studying the error correcting properties of the fermionic sector of various large N theories. Specifically we examine SU(N) matrix quantum mechanics and 3-rank tensor O(N)^3 theories. Both of these theories contain large gauge groups. We argue that gauge singlet states indeed form a quantum error correcting code. Our considerations are based purely on large N analysis and do not appeal to a particular form of Hamiltonian or holography.

10/02/2020: Presented by Dr. John Donoghue from the Amherst Center for Fundamental Interactions at the University of Massachusetts, Amherst, the title of his talk is, “Cutoffs and Gravity.”

Abstract: I will discuss issues contained in my recent papers related to using momentum space cutoffs in discussing the cosmological constant and also critiquing the present practice of Asymptotic Safety. These actually are related!

9/25/2020: Presented by Dr. Raymond Co from the William I. Fine Theoretical Physics Institute at the University of Minnesota, the title of his talk is, “New Roles of the QCD Axion in Dark Matter and Baryogenesis.”

Abstract: We propose a paradigm where QCD axion’s rotation in the potential gives rise to the dark matter abundance and the observed baryon asymmetry. The rotation is initiated by explicit Peccei-Quinn symmetry breaking effective in the early Universe. Axion dark matter is produced by the kinetic misalignment mechanism as a result of axion’s rotation. With the aid of the Standard Model sphaleron processes (and optionally the neutrino Majorana mass term), the Peccei-Quinn charge associated with the rotation is transferred to the baryon asymmetry. The paradigm predicts 1) a QCD axion heavier than predicted by the conventional evolution and 2) an electroweak phase transition temperature higher than in the Standard Model (or instead the presence of the neutrino Majorana mass).

9/18/2020: Presented by Dr. Shirley Li from the SLAC National Accelerator Laboratory in California, the title of her talk is, “Challenges in Modern Neutrino Oscillation Experiments.”

Abstract: The Deep Underground Neutrino Experiment (DUNE) will be the leading next-generation particle project in the US. It aims to measure CP violation in the neutrino sector and determine the mass ordering of neutrinos. These measurements are straightforward conceptually but challenging practically. One outstanding issue is the modeling of GeV neutrino-nucleus interaction. With a lack of a proper theoretical framework, it is not only difficult to simulate neutrino events in the detector accurately, but also difficult to assess its impact on the physics measurements. I will discuss our attempts at understanding how cross sections impact oscillation measurements and what the current theoretical uncertainties are.

Condensed Matter Theory Seminars

3/31/23: Presented by Dr. Patrick A. Lee from the Massachusetts Institute of Technology (MIT) and the California Institute of Technology (Caltech). The title of his talk is, “Strongly Driven Superconductors: What the Data tell us about Cuprates and Organics”

Abstract: In the past decade, Cavalleri's group has reported "superconducting-like" behavior up to several times Tc in HiTc cuprates, the organic superconductors and K3C60. I will review some of the data and focus on the first two systems. In collaboration with M. Michael and E. Demler, I undertook a new analysis of the YBCO data. The goal is to find the minimal set of assumptions which can explain the observations. Unlike earlier discussions, we find that intense drive does not enhance the in-plane pair correlation or the inter-bilayer correlation. However, in order to explain the data, short range order within the plane and  correlation between members within the bi-layer are required in the equilibrium state. The implication is that pairing correlation survives up to the pseudogap scale. Therefore the pseudogap is a pairing gap. For the organic superconductor, I will also discuss a proposal with Zhehao Dai that the observed enhanced gap under drive is an induced Mott gap rather than a pairing gap.

3/24/23: Presented by Dr. Liang Fu from the Massachusetts Institute of Technology (MIT). The title of his talk is, “Mott, Chern and Wigner Insulators in Semiconductor Heterostructures.”

**There were technical issues with the video recording of this seminar, so only audio is available.**

Abstract: The advent of moire superlattices in van der Waals heterostructures opens a new venue for exploring quantum phases of matter with unprecedented tunability. I will describe various kinds of quantum phases at integer fillings in semiconductor bilayer systems at zero magnetic field, including Mott and Chern insulators at the filling of one charge per unit cell, as well as Wigner solids at higher integer fillings.

1/27/23: Presented by Dr. Dan Stamper-Kurn from the University of California, Berkeley. The title of his talk is, “Many-body Physics of Atoms in Optical Lattices and Optical Cavities.”

Abstract: Ultracold atomic gases allow us to create a variety of many-body quantum systems, which we might regard as synthetic quantum materials. In some of these systems, we encounter many of the properties pertinent to condensed-matter systems. In that vein, I will present experiments on atoms in optical lattices where we test a scaling hypothesis that relates phase transitions occurring in different lattice configurations and examine transport and equilibration of itinerant particles in geometrically frustrated bands. I will also share some results obtained from experiments on atoms placed within optical cavities, where they interact strongly with an optical field. This field can serve both as a precise probe, e.g. allowing us to examine non-equilibrium thermodynamics of a mesoscopic system, and also as a dynamical component within a hybridized atom + matter quantum system. I hope to spark some discussion where we can generate some new ideas on applications of quantum simulation, on feedback-controlled quantum matter, and on phase transitions in open quantum systems.

10/17/22: Presented by Dr. Erez Berg from the Weizmann Institute in Israel. The title of his talk is, “What does the Wiedemann-Franz Law Tell Us About Non-Fermi Liquids?.”

Abstract: The Wiedemann-Franz (WF) law, stating that the Lorenz ratio L=κ/Tσ between the electronic thermal and electrical conductivities in a metal approaches a universal constant L_0 at low temperatures, is often taken to be a signature of fermionic Landau quasi-particles. In contrast, we show that various models of weakly disordered non-Fermi liquids, where the fermionic quasi-particles are either marginally defined or ill-defined, also obey the WF law at T→0. Instead, we argue that the behavior of the leading correction to the WF law at low temperature distinguishes different types of strange metals. In particular, in a solvable model of a marginal Fermi liquid, we find that the leading low-temperature deviation of L-L_0 scales as T, in contrast to a Fermi liquid where it is proportional to T^2. Moreover, by invoking a quantum Boltzmann equation approach, we demonstrate that this behavior is generic in a class of marginal- and non-Fermi liquids characterized by a weakly momentum-dependent inelastic scattering rate.

FTPI Seminars

10/10/23: Presented by Dr. Kaoru Hagiwara from KEK in Japan. The title of his talk is, “CP violating top-Higgs coupling in SMEFT.”

Abstract: The total cross section of the process mu- mu+ (greater than) vm vm~ ttH has strong dependence on the CP phase (xi) of the top Yukawa coupling, where the ratio of xi=pi and xi=0 (SM) grows to 630 at rts=30 TeV, 3300 at 100 TeV. We study the cause of the strong energy dependence and identied its origin as the (E/mW)2 growth of the weak boson fusion sub-amplitudes, WL- WL+ (greater than) ttH, when the two W's are longitudinally polarized. We repeat the study in the SMEFT framework where the EW gauge invariance is manifest, and find that the highest energy cross section is reduced to a quarter of the complex top Yukawa model result, with the same energy power. By applying the Goldstone boson (GB) equivalence theorem, we identied the origin of this strong energy growth of the SMEFT amplitudes as the contact dimension-6 pi-pi+ttH vertex, where pi^+- denotes the GB of W^+-. We obtain the Unitarity bound on the coecient of the SMEFT operator by using the optical theorem.

9/14/23: Presented by Dr. Francis Halzen from the University of Wisconsin, Madison. The title of his talk is "ICECUBE: High-energy Cosmic Neutrinos and their First Sources."

4/18/23: Presented by Dr. Junwu Huang from the Perimeter Institute in Canada. The title of his talk is, “A New Pulsar.”

Abstract: Many extensions of the Standard Model predict the presence of ultra-light bosons in the low energy theory. If any of these bosons are in the mass range of 10⁻²⁰ to 10⁻¹⁰ eV they will affect the evolution of astrophysical black holes through the superradiance process. When a boson’s Compton wavelength is comparable to the size of a black hole, the boson binds to the black hole forming a gravitational atom in the sky. The occupation number of atomic states can grow exponentially to as large as 10⁷⁶, extracting energy and angular momentum from the black hole. 
In this talk, I will present the first study of the electromagnetic signals coming from a black-hole superradiance cloud of a light dark photon. I will show how this dark photon superradiance cloud produces and hosts a rotating plasma of Standard Model charged particles. Crudely, this rotating plasma behaves like an electric dipole rotating around the black-hole spin direction. Just like a pulsar, which is qualitatively a rotating magnetic dipole, our system of a rotating electric dipole also produces extremely powerful, and potentially periodic, electromagnetic radiation. I will discuss the similarities and differences between our system and a pulsar, and several search strategies based on detailed numerical simulations.

4/14/23: Presented by Dr. Tao Han from the University of Pittsburgh in Pennsylvania. The title of his talk is, “EW Physics at Very High Energies: A Multi-TeV Muon Collider as a Case Study.”

Abstract: The Standard Model electroweak (EW) sector exhibits some novel features at very high energies. At energies much larger than the EW scale, the EW gauge symmetry is essentially restored and the massless splitting phenomena dominate the EW physics. Beyond the familiar gauge theory splitting functions, we discuss the emergence of additional ``ultra-collinear'' splitting phenomena and the naive violation of the Goldstone-boson Equivalence Theorem. Because the SU(2) quantum numbers are explicit and observable in common physical processes, subtitles of the Bloch-Nordsieck theorem violation are discussed. The vector-boson fusion processes take over as the leading contributions at high energies, and the EW parton distribution functions are formulated. We implement the EW showering and illustrate its importance by calculating a number of physical processes at high energies within and beyond the SM. Finally, we take a multi-TeV muon collider as a case study for precision Higgs physics and for discovery at the new energy frontier.

3/28/23: Presented by Dr. Robert McGehee from the University of Michigan. The title of his talk is, “Directly Detecting Light Dark Matter.”

Abstract: While the experimental program to detect ever lighter dark matter is proceeding full steam ahead, the theory of such light, detectable dark matter is at a crossroads. I will detail two examples of sub-GeV hadrophilic dark matter models which these future direct detection endeavors may discover while highlighting the serious challenges model builders face. The first achieves probe-able direct detection cross sections by way of a late-time, dark-sector phase transition, while the second does so by assuming the entire thermal bath is reheated at very low temperatures. Both models lead to dark matter-nucleon scattering cross sections of interest for near-future experiments for dark matter masses in the range of 100 keV-100 MeV, often in parts of parameter space with few or no model.

11/21/22: Presented by Dr. Nadav Outmezguine from Lawrence Berkeley National Laboratory in California. The title of his talk is, “New Physics with 21-cm Cosmology?”

Abstract: The 21-cm cosmological signal is gradually becoming a reality, offering a new insight into previously under-explored epochs. As with other cosmological observations, it is intriguing to consider what 21-cm cosmology can teach us about new physics. To address this, I will provide a concise overview of the physics behind the 21-cm cosmological signal and the effects of various new physics models on it. The discussion will then focus on hidden sector models that contain particles that scatter off baryons elastically. Specifically, I will argue that the Hydrogen Epoch of Reionization Array (HERA), aimed at measuring the 21-cm power spectrum, is likely to either confirm or exclude the possibility that DM-baryon interactions are the source of the anomalous measurement reported by the EDGES collaboration in 2018.

11/21/22: Presented by Dr. Radu Roiban from Pennsylvania State University. The title of his talk is, “An Effective Field Theory Perspective on Higher-spin Fields and the Dynamics of Spinning Bodies.”

Abstract: Gravitational interactions of higher-spin fields are of great theoretical and practical interest. On the one hand, they are a long unsolved problem strongly constrained by no-go theorems and on the other they are relevant to problems in gravitational-wave physics, where they describe spinning compact objects. In this talk we review an effective field theory approach to the classical gravitational interactions of high-spin fields and an amplitudes-based framework to integrating out gravitons and the construction of effective two-spinning-particle Hamiltonians. We present evidence for a conjectured definition of Kerr black holes from a quantum field theory perspective, identifying features that make it “the simplest” spinning body. We also discuss a possible all-order direct relation between the open-orbit observables of classical systems of two spinning particles and the eikonal phase of the four-particle scattering amplitude.

11/21/22: Presented by Dr. Kathryn Zurek from the California Institute of Technology (Caltech). The title of her talk is, “Spacetime Fluctuations from Quantum Gravity in the Infrared.”

Abstract: Quantum effects from gravity, based on naive EFT reasoning, are generally thought to decouple in the infrared. We present calculations, based on a simple prescription consistent with standard theoretical frameworks like AdS/CFT and JT gravity, where quantum gravity effects do not decouple but instead accumulate in the infrared. As a consequence, they might be observable in suitably sensitive measurements of spacetime.

4/19/22: Presented by Dr. Gordan Krnjaic from the Fermi National Accelerator Laboratory in Illinois. The title of his seminar talk is, “Towards a Realistic Model of Dark Atoms to Resolve the Hubble Tension.”

Abstract: It has recently been shown that a subdominant hidden sector of atomic dark matter in the early universe can resolve the Hubble tension while maintaining good agreement with most precision cosmological observables. However, such a solution requires a hidden sector whose energy density ratios are the same as in our sector and whose recombination also takes place at redshift z≈1100, which presents an apparent fine tuning. We introduce a realistic model of this scenario that dynamically enforces these coincidences without fine tuning. In our setup, the hidden sector contains an identical copy of Standard Model (SM) fields, but has a smaller Higgs vacuum expectation value (VEV) and a lower temperature. The baryon asymmetries and reheat temperatures in both sectors arise from the decays of an Affleck-Dine scalar field, whose branching ratios automatically ensure that the reheat temperature in each sector is proportional to the corresponding Higgs VEV. The same setup also naturally ensures that the Hydrogen binding energy in each sector is proportional to the corresponding VEV, so the ratios of binding energy to temperature are approximately equal in the two sectors. Furthermore, our scenario predicts a correlation between the SM/hidden temperature ratio and the atomic dark matter abundance and automatically yields values for these quantities that resolve the Hubble tension.

3/22/22: Presented by Dr. Mithat Ünsal from North Carolina State University. The title of his seminar talk is, “Anomaly Preserving Compactifications and Semi-classical Description of Confinement via Center-vortices.”

Abstract: I describe an anomaly-preserving compactification of four-dimensional gauge theories, including Yang-Mills theory, its supersymmetric version, and QCD, down to 2d by turning on ’t Hooft flux through a 2-torus. This provides a new framework to analytically calculate nonperturbative properties such as confinement, chiral symmetry breaking, and multi-branch structure of vacua. I give the semiclassical description of these phenomena based on the center vortices and show that it enjoys the same anomaly matching condition with the original 4d gauge theory. For YM, the long-distance theory maps to topological gauge theory (TQFT) deformed by local topological operators. We conjecture that the weak-coupling vacuum structure on small T^2 x R^2 is adiabatically connected to the strong-coupling regime in infinite volume.

3/21/22: Presented by Dr. Mithat Ünsal from North Carolina State University. The title of his colloquium talk is, “Taming Strong Dynamics and Phase Transitions.”

Abstract: Strongly coupled dynamics of QCD and other strongly coupled gauge theories remained elusive for many decades. It was believed that phenomena such as quark confinement, chiral symmetry breaking, and the emergence of the mass gap were necessarily strong coupling effects. These perspectives changed drastically over the last fifteen years. The adiabatic continuity states that strongly coupled dynamics can be continuously connected to a weak coupling regime upon judiciously formulated compactifications without any intervening phase transitions. Such continuity is impossible with thermal compactifications of gauge theories. This progress opened many new directions, leading to the discovery of many new topological excitations, confinement mechanisms, rigorous versions of semi-classical analysis, and study of calculable phase transitions. I will review these exciting developments and state some new directions.

3/22/22: Presented by Dr. Sergei Gukov from the California Institute of Technology (Caltech). The title of his seminar talk is, “Phases of 2D Minimally Supersymmetric QCD.”

3/21/22: Presented by Dr. Sergei Gukov from the California Institute of Technology (Caltech). The title of his colloquium talk is, “Building New Bridges.”

Abstract:  What do percolation and supersymmetric QCD have in common? In theoretical physics, dualities play the role of bridges that connect two different descriptions of the same physical system. Examples include the Kramers–Wannier duality in the Ising model or the AdS/CFT correspondence in string theory. When first discovered, dualities often appear surprising and mysterious, providing a strong motivation for advancing frontiers of our understanding. In this talk we will take a look at similar bridges that connect different areas of physics or, more generally, provide deep meaningful connections between completely different disciplines.

12/17/21: Presented by Dr. Subir Sachdev from Harvard University in Massachusetts. The title of his talk is, “Planckian Metals with Random Interactions.” It was recorded during the workshop, Open Concepts in the Theory of Strongly Correlated Electron Systems (OCTSCES 2021), hosted by FTPI.

11/30/21: Presented by Dr. Andrey Shkerin from FTPI, University of Minnesota. The title of his talk is, “Black Hole Induced False Vacuum Decay from First Principles.”

Abstract: We will discuss a method to calculate the rate of false vacuum decay induced by a black hole. The method uses complex tunnelling solutions and consistently takes into account the structure of quantum vacuum associated to the black hole. We will illustrate the technique on a two-dimensional toy model of a scalar field with inverted Liouville potential in an external background of a dilaton black hole. Using this model, we will compute the exponential suppression of tunnelling from the Boulware, Hartle-Hawking and Unruh vacuum states and show that they are parametrically different. Finally, we will discuss how our results are generalized to the realistic case of black holes in four dimensions.

10/25/21: Presented by Dr. Alex Maloney from McGill University in Canada. The title of his talk is, “Gravity as an Average.”

Abstract: I will explore the idea that certain theories of gravity are not standard quantum mechanical theories, but are instead ensemble averages of many quantum theories. This is inspired by connections between black holes, wormholes, chaos, and the dynamics of disordered systems. To study this, we will need to understand ensembles of random field theories which generalize random matrix theory. I will describe are relatively simple version of this problem which involves averages over free boson CFTs in two dimensions. In this case the ensemble average precisely reproduces the sum over geometries of an exactly solvable (but somewhat exotic)theory of quantum gravity. This is a completely solvable model of holography,and has important implications for our understanding of quantum black holes.

4/27/21: Presented by Dr. Sandipan Kundu from Johns Hopkins University in Maryland. The title of his talk is, “Causality, Higher-Spin Particles, and Large N QCD.”

Abstract: I will show that metastable higher-spin particles, free or interacting, cannot couple to gravity while preserving causality unless there exist higher spin states in the gravitational sector much below the Planck scale. Causality imposes an upper bound on the mass of the lightest higher spin particle in the gravity sector in terms of quantities in the non-gravitational sector. I will argue that any weakly coupled UV completion of such a theory must have a gravity sector containing infinite towers of asymptotically parallel, equispaced, and linear Regge trajectories. This implies that the gravity sector has a stringy structure with an upper bound on the string scale. I will discuss many surprising implications of the above bound for large N QCD coupled to gravity.

4/20/21: Presented by Dr. Mithat Ünsal from North Carolina State University. The title of his talk is, “Resurgence, Lefschetz Thimbles and Hidden Topological Angles.”

Abstract: I will describe aspects of resurgence in quantum mechanics and Yang-Mills theory. In three quantum mechanical systems with the same classical (double well) potential, non-supersymmetric, N=1 supersymmetric, and N=2 (extended) supersymmetric, all of which possess the same instanton solution, I will show how thinking about Lefschetz thimbles produces distinct and correct result in each case. I will provide evidence that many features of semi-classical expansion generalize to 4d YM theory. By thinking about calculable semi-classics with fractional instantons, I will show that the quantum mechanical discussion generalizes to non-supersymmetric and supersymmetric YM. If time permits, I will briefly describe coupling YM theory to a topological theory (TQFT) on a four torus, and the type of new quantum mechanical systems that emerge in the Born-Oppenheimer approximation.

FTPI Faculty Seminars and Interviews

FTPI Professor Maxim Pospelov delivered this Cosmology Seminar on 11/28/22. The title of his talk is, "Accelerated Light Dark Matter."

Abstract: Light dark matter particles (MeV range) carry too little energy to be seen with current dark matter detectors. Considerable efforts are being spent on lowering the energy thresholds for detection. I will show that these models can be successfully constrained using a double collision scheme. First collision with e.g. solar electrons/cosmic ray particles/accelerator beam can bring the dark matter to detectable energies, with subsequent scattering of accelerated dark matter leaving detectable energy deposition in large neutrino and/or dark matter detectors. This scheme currently provides leading sensitivity to light dark matter in a wide range of masses and cross sections.

FTPI Professor Mikhail "Misha" Shifman delivered this Seminar on 6/17/21. The talk was hosted by the Harvard University Center of Mathematical Sciences and Applications. The title of his talk is, "What can supersymmetry do that other field theories cannot."

An interview with FTPI Professor Mikhail "Misha" Shifman, "50 Years in Theoretical Physics."

Hosted by Suzette Lyn

FTPI Professor Mikhail "Misha" Shifman delivered this Special Seminar on 3/1/21. The title of his talk is, “Quantum Physicists Between Two Evils.”

FTPI Professor Keith Olive was interviewed on the podcast, Scientific Sense on Oct. 6th, 2020. Listen to his interview now.

Larkin Award Colloquiums

4/30/23: 2022 Anatoly Larkin Senior Award

Professor Patrick A. Lee from the Massachusetts Institute of Technology (MIT) and the California Institute of Technology (Caltech) was awarded the Senior Award for pioneering and wide reaching research in strongly correlated systems, in particular theories of the quantum transport phenomena in the mesoscopic and superconducting systems, and for his standing in the Physics community.

The title of his talk is, "An Overview of Quantum Spin Liquid: Standing on the Shoulder of a Giant Named Anatoly."

Abstract: I shall review the current status of quantum spin liquid, particularly the gapless variety which is described by emergent gauge fields and fermionic particles called spinons. The theory has benefited greatly by the insight of Anatoly Larkin on the role of gauge field fluctuations, leading to the famous Ioffe-Larkin rule. I shall review the status of several promising experimental candidates and describe some proposals to experimentally access the spinons as well as the gauge field.

4/23/23: 2022 Anatoly Larkin Junior Award

Professor Liang Fu from the Massachusetts Institute of Technology (MIT) was awarded the Junior Award for seminal works on 3D topological insulators and odd parity topological superconductors, crystalline topological insulators, Majorana zero modes, and for being an intellectual leader of his generation.
The title of his talk is, “Diodic Quantum Materials.”

Abstract: The p-n junction is the key building block of modern microelectronics that underlies diodes and transistors. In recent years, it has been found that certain quantum materials can have a direction dependent electrical resistance and thus exhibit an intrinsic diode effect without any junction. In this talk, I will first describe diodic superconductors that exhibit zero (nonzero) resistance in the forward (backward) direction. Such superconducting diode effect generally appears when Cooper pairs in the ground state have finite center-of-mass momentum, as in the Larkin-Ovchinnikov-Fulde-Ferrell superconductor. Next, I will describe noncentrosymmetric conductors that exhibit a nonreciprocal Hall effect at zero magnetic field, with the transverse current quadratic in the applied voltage. This intrinsic nonreciprocity is a fundamental material property that originates from the quantum geometry of itinerant electron states in crystals. Potential applications of diodic quantum materials in high-frequency (THz) and low-power electronics will be discussed.