# Recorded Seminars

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

**FTPI Seminars | High Energy Lunchtime Seminars | FTPI Faculty Interviews & Seminars**

## FTPI Seminars

### 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.*

## High Energy Theory Lunchtime Seminars

## Jump to Year: 2022 | 2021 | 2020

### 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 Quebec. 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 England. 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.*