From critical phenomena to quantum gravity, conformal field

theories (CFTs) describe the universal scale-invariant structures that lie at

the heart of theoretical physics. The conformal bootstrap is the

powerful idea, dating back to the 70’s, that one can use fundamental

consistency conditions to constrain, solve, and map out the space of

conformal field theories. In this talk I will describe recent progress in using

the conformal bootstrap to perform precise calculations and chart the

landscape of 3d CFTs involving interacting scalars, fermions, and gauge fields.

**Special Seminar** Please note time and location

In the summer of 1925, Heisenberg wrote the paper Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen, which laid the foundations of quantum mechanics. For a long time, this paper was considered to be inscrutable. This talk will show how one can make sense both of Heisenberg's formal manipulations and of his philosophical rhetoric, in particular by studying the letters he wrote in months leading up to his breakthrough. A particular emphasis will be placed on how different the theory that Heisenberg originally aimed to construct was from modern quantum mechanics

The search for dark matter with masses between meV-to-GeV has seen tremendous theoretical and experimental progress in the past few years. I will provide an overview of some of this progress. I will mention recent results from SENSEI, a Skipper-CCD-based experiment that is sensitive to (halo) dark matter scattering off electrons for masses larger than ~1 MeV. I will highlight how such detectors can, however, also probe sub-MeV dark matter masses by searching for the component of dark matter that is boosted to higher energies from scattering in the Sun. In particular, future detectors with larger exposures could probe the entire “freeze-in” benchmark model down to keV masses. I will also discuss some challenges for direct detection, in particular, novel low-energy backgrounds. I will introduce a new detector concept, the “dual-sided CCD”, which could help with distinguishing some of these backgrounds.

]]>**Please note special date, time and location for this seminar**

NN Khuri, in 1995, considered a potential model where one spatial

dimension was compactified. He adopted Green function method, which has been

used for case of noncompact potential model, to study analyticity property

of forward amplitude and showed it violates dispersion relation (in case of

noncompact potential model dispersionrelation had been proved in 1950's). If

Khuri's result were valid in relativistic QFT it will have serious

consequences (for example proof of Frossart bound will not be valid). Andre

Martin asked me to look at analyticity of amplitude for a theory with

compactified spatial dimension.

I studied analyticity for such a theory in the framework of general field

theory without adopting any specific model. I have proved two results (i)

Forward dispesion relation is valid in relativistic QFT for a massive scalar

field theory with a compact spatial dimention. Thus Khiri's result for

potential scattering does not hold good for relativistic QFT. (ii) I have

gone further; I proved nonforward dispersion relation. Thus I

have gone beyond Khuri.

The count of black hole microstates is typically obtained from a supersymmetric index in weakly coupled string theory. I will discuss the index in the strongly coupled theory, as a functional integral in N=2 supergravity in asymptotically flat space. The saddle-points of this index are given by supersymmetric "finite-temperature" rotating geometries. I will discuss a new version of the attractor mechanism obeyed by these geometries: the scalar fields at the poles of the Euclidean horizon as well as the free energy of the black hole get attracted to values that depend only on the charges and are independent of the asymptotic moduli and temperature.

]]>Effective field theories (EFTs) suffer from a vast redundancy of description, reminiscent of coordinate invariance, that lends itself to a geometric treatment. In this talk I’ll survey recently-developed geometric insights into EFTs of the Standard Model Higgs sector, including invariant distinctions between possible EFTs of the Higgs boson, a new understanding of the connection between EFT geometry and observables, and generalizations of Riemannian field space geometry that encode information about analyticity and unitarity of the EFT. These developments are relevant to ongoing searches at the LHC and sharpen an open question for future colliders: is electroweak symmetry linearly realized by the known particles of the Standard Model?

]]>I will begin by reviewing the movitations for studying the TT* deformation of two dimensional field theories, the original formulation of this deformation, and its formulation in terms of Jackiw-Teitelboim gravity. Then I will discuss how to compute correlation functions of local operators using this formulation, in which the position of the operators is defined using the dynamical coordinates of the formalism. I will focus on the large-momentum behavior of the two-point function when the undeformed theory is a conformal field theory. The main result (based on 2304.14091) is that for momentum q it is given by |q|^{-q^2 t/\pi}, where t is the deformation parameter. Interestingly, the sign of the exponent is different than previous computations which resummed the small momentum expansion. The decay at large momentum manifests the non-locality of the theory, which also appears through the fact that operators with different momentum require a different multiplicative renormalization, and that the large-momentum behavior of the correlation function on the torus is different from the behavior mentioned above on the plane.

]]>Inspired by the second law of thermodynamics, we study the change in subsystem entropy generated by dynamical unitary evolution of a product state in a bipartite system. Working at leading order in perturbative interactions, we prove that the quantum n-Tsallis entropy of a subsystem never decreases, provided that subsystem is initialized as a statistical mixture of states of equal probability. This is true for any choice of interactions and any initialization of the complementary subsystem. When this condition on the initial state is violated, it is always possible to explicitly construct a "Maxwell's demon'' process that decreases the subsystem entropy. Remarkably, for the case of particle scattering, the circuit diagrams corresponding to n-Tsallis entropy are the same as the on-shell diagrams that have appeared in the modern scattering amplitudes program, and the entropy growth is intimately related to the nonnegativity of cross-sections.

]]>Averaged null energy, defined by integrating the null energy over a light ray, is known to be closely tied to causality in AdS/CFT, to deformations of the modular Hamiltonian in quantum field theory, and to the Lorentzian inversion formula in CFT. I will discuss a new connection between averaged null energy and the monotonicity of the renormalization group, and use the averaged null energy condition (ANEC) to derive the c-theorem in two dimensions and the a-theorem in four dimensions. The derivation is based on contact terms that appear in correlation functions of the light-ray operator. Combined with previous results, this also gives a new derivation of the c- and a-theorems from the monotonicity of relative entropy, and hints at a more general role for Lorentzian inversion in non-conformal QFTs.

]]>The black hole information problem describes a tension between the point of view of an observer exterior to a black hole, who should see it evaporate according to the laws of quantum mechanics, and an observer who falls in, who should see nothing special about spacetime as they pass the event horizon. The experience of the infalling observer must be encoded in the fundamental description of the black hole; such a “holographic map” must be inherently non-isometric. I describe the “backwards-forwards map”, a candidate holographic map involving the time evolution of both the interior and exterior descriptions as well as post-selection, in a toy model for a black hole made out of qubits.

]]>In the era of a wealth of data from the sky, new perspectives can lead to parametric improvements in discovery reach. I will discuss two ideas that make use of surprising properties of coherent radiation to open new directions for detection

First is intensity interferometry, which relies on the second-order coherence of light. By recording photon counts rather than electromagnetic fields at a telescope, intensity interferometry admits longer baselines in the optical and thus greater precision than traditional interferometry. I will describe the Extended-Path Intensity Correlator (EPIC): a proposed telescope array that extends the scope of intensity interferometry. Combined with advances in spectroscopy and single-photon detection, EPIC can achieve unprecedented precision in astrometry with applications including exoplanet detection and black hole measurements.

Second is superradiance: stimulated emission of radiation from an absorbing body. I will discuss how rotating black holes, through the process of superradiance, become laboratories in the sky for ultralight bosons including the elusive QCD axion. When a boson's Compton wavelength is comparable to the horizon size of a black hole, the black hole spins down and converts energy into an exponentially growing cloud of bosons. Depending on the bosons' interactions, the resulting systems can be visible across the spectra: emitting gravitational wave radiation, populating the galaxy with axion waves, or appearing as novel pulsar-like objects in the sky.

Black holes with two charges in string theory are singular due to vanishing horizon area at extremality. Two seemingly contradictory resolutions are available in the literature. On one hand, it has been argued that higher-derivative effects create a string-sized extremal horizon.

On the other hand, it has been argued that before such a small black hole even forms, there is a transition to a winding condensate and eventually a gas of strings. We show that, with some modifications, these two perspectives are compatible, but correspond to different observables.

A rotating non-extremal black hole solution with higher-derivative corrections contributes to the gravitational path integral for the index,

while the transition to the gas of strings happens for the thermal partition function. Along the way, we extend the recently developed ``new attractor mechanism" by incorporating higher-derivative corrections. We also use effective theory to rule out the possibility of a string-size black hole as the correct description of the near-extremal microstates.

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

]]>We live in remarkable times: the recent advent of gravitational-wave observations allows testing gravity in a strongly relativistic regime. We also have plausible candidates for UV physics that reconciles General Relativity with Quantum Mechanics. But there is also Bad News: Decoupling - which beautifully explains why low-energy measurements are largely insensitive to UV details - seems a central organizing feature of Nature that thwarts the extraction of fundamental insights about UV physics from astrophysical or cosmological observations. This talk argues that all is not lost because some UV features can penetrate the decoupling barrier in interesting ways. In particular generic accidental symmetries can robustly point to the existence of scalars in the low-energy effective theory (and these are not just axions). Normally we are taught that naturalness arguments preclude these scalars from being light enough or too weakly coupled to be important for tests of gravity, but I argue that the additional information that the observed Dark Energy is so small puts us in a regime where some scalars are pseudo-dilatons (ie naturally light with Brans-Dicke couplings to matter). The question of why these scalars are not already detected motivates more detailed studies of whether screening mechanisms exist that could have hidden them from present-day tests of gravity. Crucially they must do so in a way consistent with other properties of UV completions of gravity (in a way that standard screening mechanisms - like Chameleons - are not). The talk describes new proposals for such UV-consistent screening mechanisms and why they thread a blind spot in current theoretical approaches to testing gravity. If time permits I will also explore other implications these models might have, including possible relevance to other problems like the Hubble tension.

]]>Critical Phenomena, including the appearance of universal scaling laws and critical exponents in the vicinity of phase transitions, appear in different fields of physics and beyond. Critical phenomena in gravitational collapse to black holes were first observed by Matt Choptuik 30 years ago - a seminal discovery that launched an entire new field of research. While these phenomena are well understood in spherical symmetry, critical collapse of gravitational waves has remained elusive. In this talk I will review the appearance of scaling laws and self-similarity close to the onset of black hole formation, and will then present simulations of gravitational-wave collapse with three independent numerical codes. These results strongly suggest that the threshold solution for vacuum collapse is not universal, and that our understanding of critical collapse in the absence of spherical symmetry will have to be broadened.

]]>In massless QED, we find that the classical U(1) chiral symmetry is not completely broken by the Adler-Bell-Jackiw anomaly. Rather, it is resurrected as a generalized global symmetry labeled by the rational numbers. Intuitively, this new global symmetry in QED is a composition of the naive axial rotation and a fractional quantum Hall state. The conserved symmetry operators do not obey a group multiplication law, but a non-invertible fusion algebra. We further generalize our construction to QCD, and show that the neutral pion decay can be derived from a matching condition of the non-invertible global symmetry.

]]>The most important experimental probes of fundamental physics involve

the scattering of elementary particles. Over the years we have seen

significant progress in understanding the properties of scattering

amplitudes and in our ability to carry out new computations both for

theoretical and phenomenological purposes. I will overview some recent

developments in N=4 Yang-Mills amplitudes.

Causality conditions provide powerful constraints on low energy theories. In this talk, I will discuss how standard causality conditions of AdS and flat spacetimes can be extended to de Sitter spacetime. In particular, I will consider the Shapiro time delay experienced by a particle in a black hole or shockwave background and discuss how "fastest null geodesics" can be defined using spatial shifts on the boundary of de Sitter and the relevance of the "stretching" of the de Sitter Penrose diagram. I will discuss a few illustrative examples.

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