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: 2021 | 2020
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.