High Energy Physics

Pulsar magnetospheres admit non-stationary vacuum gaps that are characterized by non-vanishing E•B. These gaps play an important role in plasma production and electromagnetic wave emission and, as I will discuss, are very efficient axion factories. The density of axions produced in a vacuum gap can be several orders of magnitude greater than the ambient dark matter density.  In the strong pulsar magnetic field, a fraction of these axions may convert to photons, giving rise to broadband radio signals. We show that dedicated observations of nearby pulsars with radio telescopes (FAST) and interferometers (SKA) can probe axion-photon couplings that are a few orders of magnitude lower than current astrophysical bounds.

Non-gaussianity of primordial density perturbations can be sensitive to very heavy particles at the inflationary Hubble scale (H < 10^(13) GeV). However, the window of observability is often constrained to masses close to H. In this talk, I will discuss a mechanism (dubbed “chemical potential”) for heavy complex scalar fields that can extend this window to masses as large as 60H. The mechanism utilizes the large kinetic energy of the inflaton to enhance particle production, and can impart observable non-gaussianity, f_NL~ O(0.01-10). In the second part of the talk, I will discuss another mechanism where the distinct signature of a heavy field can be imprinted at the level of the power spectrum by violating scale-invariance. This can be achieved through the onset of classical oscillations of the heavy field during inflation, instead of quantum production. We consider the possibility of observing such a signal in the stochastic gravitational wave (GW) background originating from a first-order phase transition in a hidden sector. The signal can be observably large in the GW map while being completely hidden in the standard curvature perturbations such as those of the CMB.

Recent studies revealed that wormhole geometries play a central role in understanding quantum gravity. After disorder-averaging over random couplings, Sachdev-Ye-Kitaev (SYK) model has a collective field description of wormhole saddles. A recent paper by Saad, Shenker, Stanford, and Yao studied the SYK model with fixed couplings and found that the wormhole saddles persist, but that new saddles called “half-wormholes” also appear in the path-integral. 

In this talk, we introduce a “partially disorder-averaged SYK model” and study how these half-wormholes emerge as we gradually fix the coupling constants. This model has a real parameter that smoothly interpolates between the ordinary totally disorder-averaged SYK model and the totally fixed-coupling model. For the large N effective description, in addition to the usual bi-local collective fields, we also introduce a new additional set of local collective fields. These local fields can be understood as the “half” of the bi-local collective fields, and they represent the half-wormholes in the totally fixed-coupling limit. We found that the large N saddles of these local fields vanish in the total-disorder-averaged limit, while they develop non-trivial profiles as we gradually fix the coupling constants. This illuminates how the half-wormhole saddles emerge in the SYK model with fixed couplings.

We study the Lyapunov exponent in disordered quantum field theories. Generically the Lyapunov exponent can only be computed in isolated CFTs, and little is known about the way in which chaos grows as we deform the theory away from weak coupling. In this talk we describe families of theories in which the disorder coupling is an exactly marginal deformation, allowing us to follow the Lyapunov exponent from weak to strong coupling. We find surprising behaviors in some cases, including a discontinuous transition into chaos. We also describe a new method allowing for computations in nontrivial CFTs deformed by disorder at leading order in 1/N.

When a generic isolated quantum many-body system is driven out of equilibrium, its local properties are eventually described by the thermal ensemble. This picture can be intuitively explained by saying that, in the thermodynamic limit, the system acts as a bath for its own local subsystems. Despite the undeniable success of this paradigm, for interacting systems most of the evidence in support of it comes from numerical computations in relatively small systems, and there are very few exact results. In the talk, I will present an exact solution for the thermalization dynamics in the "Rule 54" cellular automaton, which can be considered the simplest interacting integrable model. After introducing the model and its tensor-network formulation, I will present the main tool of my analysis: the space-like formulation of the dynamics. Namely, I will recast the time-evolution of finite subsystems in terms of a transfer matrix in space and construct its fixed-points. I will conclude by showing two examples of physical applications: dynamics of local observables and entanglement growth. The talk is based on a recent series of papers: arXiv:2012.12256, arXiv:2104.04511, and arXiv:2104.04513.

Thanks to integrability, we have access to a significant portion of the conformal data of planar N=4 SYM: in particular, the scaling dimensions of all single-trace operators at finite 't Hooft coupling. Can we solve the theory by using these data as an input for the conformal bootstrap? 

We recently started to explore this question in a simplified setup: the one-dimensional CFT living on a supersymmetric Wilson line embedded in the 4D theory. 

After reviewing how integrability describes the spectrum of this 1D CFT, I will discuss how, using the numerical conformal bootstrap with a minimal input from the spectral data, it is possible to compute with good precision a non-supersymmetric OPE coefficient at finite 't Hooft coupling. I will conclude discussing the open questions and future perspectives. Based on 2107.08510 with N.Gromov, J.Julius and M.Preti.

Sterile neutrinos have been proposed to tackle a number of outstanding questions in physics, including the phenomena of dark matter, neutrino masses and the baryon asymmetry of the Universe. I will review current limits on GeV-, MeV- and keV-scale sterile neutrinos from cosmology and astrophysics. In particular, I will focus on how primordial abundance determinations, Cosmic Microwave Background observations and stellar kinematic inferences from dwarf spheroidal galaxies allow us to set robust constraints across this mass scale. I will then end with a short discussion on how sterile neutrinos could relax cosmological bounds on neutrino masses and what implications this would have for experiments like KATRIN.
 

In this talk, we explore new techniques to probe the analytic structure of scattering amplitudes in perturbative Quantum Field Theory. The goal of this approach is to leverage symmetries, limits, and analyticity in order to circumvent the explicit evaluation of multi-loop Feynman integrals. First, we generalize the cutting rules by relating sequential discontinuities (discontinuities of discontinuities) to multiple cuts through the corresponding Feynman diagram. Then, we determine the logarithmic branch-cut structure of massive scattering amplitudes by expanding around their branch points. As a corollary, we prove a conjectured bound on the transcendental weight of polylogarithmic Feynman integrals. These results pave a new path towards bootstrapping scattering amplitudes in perturbation theory.

Finding cosmological models that are consistent with a wide range of observations, including those that indicate a high Hubble constant (the current rate of expansion of space), has proved to be very challenging. In this talk I explore why. Our exploration leads us to fundamental sources of length scales in cosmological models: gravitational collapse time scales and photon electron-scattering mean free paths. We find that scaling down these quantities leaves cosmic microwave background (CMB) maps, and many other cosmological observables, nearly invariant, while boosting up the expansion rate. I then introduce a model that takes advantage of this symmetry to boost up model predictions of the Hubble constant given CMB and other cosmological data. Gravitational collapse time scales are scaled down from standard cosmological values by the introduction of a `mirror world' dark sector, and photon mean free paths are scaled down by reducing the amount of helium, thereby freeing up more electrons. I speculate about alternative ways to reduce photon mean free paths, as consistency with the Riess et al. (2021) measurement of the Hubble constant requires there to be less helium in the universe than is observed. 

The universe has been studied using light since the dawn of astronomy.  

But deep down in the dark glacial ice of the South Pole, Antarctica, a very different kind of telescope is getting a new view of the universe. Operated by a team of more than 300 physicists from 12 countries, the IceCube Neutrino Observatory captures the universe in high-energy neutrinos.  

Neutrinos are particles a lot like light (photons), but with one remarkable property that makes them a powerful medium for studying the universe. Physicist Naoko Kurahashi Neilson has travelled to the snow-swept IceCube Neutrino Observatory to study these elusive particles. In her October 6 Perimeter Public Lecture webcast, she will share more about the insights neutrinos can offer and what it’s like conducting research in one of the least habitable places on Earth.

Kurahashi Neilson is an associate professor at Drexel University and the recipient of a CAREER award from the National Science Foundation. Symmetry magazine featured her among 10 early-career experimentalists of note in 2019.

After her undergraduate degree from University of California, Berkeley, Kurahashi Neilson obtained her PhD at Stanford University while “listening” for extremely high-energy neutrinos in the ocean in the Bahamas. She now lives outside Philadelphia with her husband and three young children, and is devoted to STEM outreach, particularly aimed at middle- and high-school girls.

In this talk, I will connect physical properties of scattering amplitudes to the Riemann zeta function. Specifically, I will construct a closed-form amplitude, describing the tree-level exchange of a tower with masses m^2_n = \mu^2_n, where \zeta(\frac{1}{2}\pm i \mu_n) = 0. Requiring real masses corresponds to the Riemann hypothesis, locality of the amplitude to meromorphicity of the zeta function, and universal coupling between massive and massless states to simplicity of the zeros of \zeta. Unitarity bounds from dispersion relations for the forward amplitude translate to positivity of the odd moments of the sequence of 1/\mu^2_n.

Cosmic inflation provides an environment similar to particle colliders that can produce new particles and record the resulting signal. In this talk, I will describe a scenario in which new particles much heavier than the Hubble scale are produced during inflation via couplings to the inflaton. These heavy particles propagate classically and give rise to localized spots on the cosmic microwave background following their production. Momentum conservation during particle production dictates that these localized spots come in pairs. I will discuss the properties of such pairs of CMB spots and the prospect of their detection from the thermal fluctuation background in a position space search.