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Winter 2019

Jack Collins (Maryland) :  CWoLa Hunting -- Mmachine Learning for model-agnostic bump hunts | 1/16/19

New physics at the LHC would typically manifest as an anomalous overdensity of events in some phase space region of the high-dimensional feature space of LHC data. The traditional way to search for new physics is to make some theory-motivated guess as to what it  will look like, and then make a phase space selection which is optimized using simulated data and then look in that region for an excess in the real LHC data. Higher sensitivity is often achieved at the expense of introducing stronger assumptions about the underlying signal model, which are used to make more optimised multivariate cuts using more event features. I will discuss a case study of an alternate paradigm, in which sensitive multivariate selections can be be found while maintaining few signal-model assumptions and without the need for potentially unreliable signal simulations. The key ingredient is a machine learning algorithm which searches for event over-densities on an otherwise smooth background, as is often the case in bump hunts for particle resonances. In this 'CWoLa-hunting' (Classification Without Labels) strategy, the selection cuts are not  determined in advance but are rather dictated by the distribution of the actual measured LHC data. I will also provide a summary of some of the other ideas for using machine learning for model-agnostic searches that have been proposed in 2018.


Pranjal Nayak (U Kentucky, Lexington) : Analytic approach to Eigenstate thermalization (ETH) in the SYK model and the Schwarzian theory | 1/23/19

The SYK model provides an uncommon example of a theory where Eigenstate Thermalization Hypothesis (ETH) can be verified in analytically. In this talk I will discuss this model in the deep infrared limit where the theory has an emergent conformal (reparametrization) symmetry that is broken both spontaneously and explicitly. To study the validity of ETH, we compute the heavy- light correlation functions of operators in the conformal spectrum of the theory. We compute these correlation functions with and without the contribution of the low energy (Schwarzian) modes, which are known to be the origin of the chaotic behaviour in this theory. In considering the contributions of the Schwarzian modes we find a weaker form of ETH: while the heavy operator insertions increase the effective temperature perceived by the light insertions, this effective temperature is proportional to the background temperature and goes to zero with the background temperature. In the case where Schwarzian modes aren’t considered, we find ETH in limit in which the weight of the heavy operators approach infinity. I will also discuss
implications of these results for the states in AdS2 gravity dual.


Oren Slone (PCTS, Princeton) : Testing Models of Dark Matter and Modifications to Gravity using Local Milky Way Observables | 1/30/19 *(cancelled snow storm)

Galactic rotation curves are often considered the first robust evidence for the existence of dark matter. However, even in the presence of a dark matter halo, other galactic-scale observations, such as the Baryonic Tully-Fisher Relation and the Radial Acceleration Relation, remain challenging to explain. This has motivated various models of dark matter as well as long-distance, infrared (IR) modifications to gravity as an alternative to the dark matter hypothesis. We present a framework to test a general class of such models using local Milky Way observables, including the vertical acceleration field, the rotation curve, the baryonic surface density, and the stellar disk profile. In this talk I will focus on models that predict scalar amplifications of gravity, i.e., models that increase the magnitude but do not change the direction of the gravitational acceleration. MOdified Newtonian Dynamics (MOND) as well as superfluid dark matter are examples. We find that models of this type are in tension with observations of the Milky Way scale radius and bulge mass and that cold non-interacting dark matter provides a better fit to the data. We conclude that models that result in a MOND-like force struggle to simultaneously explain both the rotational velocity and vertical motion of nearby stars in the Milky Way. A future publication will extend this analysis to include other models such as Strongly Interacting Dark Matter (SIDM).

Matthijis Hogervorst (Perimeter) : Hamiltonian truncation and the S^3 partition function | 2/6/19

In this talk I discuss Hamiltonian truncation, a toolkit to construct quantum field theories. Hamiltonian truncation is in many ways orthogonal to the more familiar lattice regularization, and it can be used to systematically compute QFT observables with little computational effort. In the first part of this talk I will review the basic ideas behind this method, as well as some examples from the literature in d=2 and d>2 dimensions. In the second part I will discuss recent work involving strongly-coupled scalar theories on the three-dimensional sphere. Based on hep-th/1811.00528.


Raffaele D'Agnolo (Stanford) : Learning New Physics from a Machine | 2/13/19

I will discuss how to use neural networks to detect data departures from a given reference model, with no prior bias on the nature of the new physics responsible for the discrepancy. The algorithm that I will describe returns a global p-value that quantifies the tension between the data and the reference model. It also allows to compare directly what the network has learned with the data, giving a fully transparent account of the nature of possible signals. The potential applications are broad, from LHC physics searches to cosmology and beyond.

Huajia Wang (KITP at UC Santa Barbara) : energy condition, modular flow, and AdS/CFT | 2/20/19

In recent years, substantial progresses has been made in understanding and proving a number of energy conditions in quantum field theories (QFTs), which played very important roles for constraining quantum corrections to black hole dynamics in general relativity. In this talk, I will discuss proof of the quantum null energy condition (QNEC), both in holographic CFTs based on AdS/CFT, and in generic CFTs using techniques related to the  entanglement structure. Furthermore, I will discuss the connection between the two approaches, and in doing this, deep relations between boundary modular flow and bulk RT surface dynamics will be revealed.


Jessica Turner (Fermilab) : Searching for flavour symmetries: old data new tricks | 2/27/19

The observed pattern of mixing in the neutrino sector may be explained by the presence of a non-Abelian, discrete flavour symmetry broken into residual subgroups at low energies. These flavour models require the presence of Standard Model singlet scalars, namely flavons, which decay to charged leptons in a flavour-conserving or violating manner. In this talk, I will present the constraints on the model parameters of an A4 leptonic flavour model using a synergy of g-2, charged lepton flavour conversion and collider data. The most powerful constraints derive from the MEG collaboration's result and the reinterpretation of an 8 TeV ATLAS search for anomalous productions of multi-leptonic final states.


Temple He (UC Davis) : Asymptotic Symmetries and the Soft Photon Theorem in Arbitrary Dimensions | 3/13/19

We show that Weinberg's leading soft photon theorem in massless quantum electrodynamics (QED) implies the existence of an infinite-dimensional large gauge symmetry, which acts non-trivially on the null boundaries of (d+2)-dimensional Minkowski spacetime. These symmetries are parameterized by an arbitrary function of the d-dimensional celestial sphere living at null infinity. This extends the equivalence between Weinberg’s leading soft photon theorem and the large gauge symmetries of QED from even dimensions higher or equal to four to all dimensions.


Jeff Dror (UC Berkeley) : Pulsar timing as a probe of primordial black holes and subhalos | 3/20/19

Pulsars act as accurate clocks, sensitive to gravitational redshift and acceleration induced by transiting clumps of matter. In this talk, I study the sensitivity of pulsar timing arrays (PTA) to transiting compact dark matter objects, focusing on primordial black holes and subhalos. Such dark matter clumps can result in different classes of signals observable in pulsar timing experiments depending on the mass of the object. I will classify the types of signals, where they are most important, and the different search strategies resulting in possible constraints over a huge mass range, 10^−12 to 100 solar masses. Crucially, PTAs offer the opportunity to probe much less dense objects than lensing experiments due to the large effective radius over which such objects can be observed with a single pulsar. We project the reach possible with current and future pulsar timing experiments, with sensitivity to a dark matter sub-component reaching the sub-percent level over significant parts of this range with future detectors.


Dalimil Mazac (Stonybrook) : Sphere packing and quantum gravity | 3/27/19

The sphere packing problem asks to find the densest possible packing of identical spheres in d dimensions. The problem was recently solved analytically in 8 and 24 dimensions by Viazovska et al., building on linear programming bounds of Cohn+Elkies. I will show that there is a close connection between these results on sphere packing and the modular bootstrap in two-dimensional conformal field theories. In particular, I will explain that Viazovska's solution was essentially rediscovered in the conformal bootstrap literature in the guise of "analytic extremal functionals". It corresponds to saturation of the modular bootstrap bounds by known 2D CFTs. Sphere packing in a large number of dimensions maps to the modular bootstrap at large central charge, which can be used to constrain quantum gravity in large AdS_3. I will use the new analytic techniques to improve significantly on the best asymptotic upper bound on the mass of the lightest state in such theories.

Julio Parra-Martinez (UCLA) : Gravity amplitudes from the ultraviolet | 4/10/19

Scattering amplitudes in planar N=4 super Yang-Mills can be described in terms a geometrical object, the Amplituhedron. Special properties of loop integrands seem to indicate that this picture persists beyond the planar limit. My talk will describe a first step, and several challenges, in finding similar structures in gravity amplitudes. I will explain how their ultraviolet behaviour, usually considered problematic, might hold the key to this problem.

Eric Perlmutter (Caltech) : Finding String Theory from the Large N Bootstrap | 1/5/19

I will discuss some recent methods for computing nonplanar CFT correlators, dual to one-loop amplitudes in AdS. This will include two applications to string theory: first, the development of a novel approach to computing perturbative string amplitudes; and second, a rigorous way to count the number of "large'' extra dimensions in the gravity dual of a strongly coupled, large N CFT.