fall2020

KCL Strings Journal Club Fall 2020

Included page "clone:kclstrings" does not exist (create it now) Fall 2020 Journal Club Here's a summary of all the activity of our Journal Club during the Fall 2020 term.

Summary

December 18: Papers discussion We will be discussing: From multi-gravitons to Black holes: The role of complex saddles (Alejandro) Geometrizing TTbar by Caputa, Datta, Jiang and Kraus (Andrea) December 16: Marco Meineri (CERN) Title: Hamiltonian truncation in AdS Abstract: We study quantum field theory in AdS_2 within the framework of Hamiltonian truncation. We start from a solvable theory and we deform the Hamiltonian with an interaction term. We find the spectrum as a function of the coupling, and we aim at connecting the solvable regime with the strongly coupled flat space physics. Hamiltonian truncation in infinite volume presents specific challenges, and special care is needed to recover the physical energy gaps as the cutoff is removed. To this end, we propose a general prescription, we give an argument for its validity and we check it in various examples. December 11: Papers discussion We discussed Five-Dimensional Non-Lorentzian Conformal Field Theories and their Relation to Six-Dimensions (Neil, Arthur, Rishi, Paul) Quantum gravity on the black hole horizon (Gaddam, Groenenboom, 't Hooft) - Kevin December 9: Gabi Zafrir (Milano-Bicocca U.) Title: An N=1 Lagrangian for an N=3 SCFT Abstract: Purely N=3 SCFTs are naturally strongly coupled, and expected to not have a Lagrangian description manifesting their N=3 supersymmetry. However, these may be reached through a deformation of a theory preserving less supersymmetry. We shall present a strategy to try to conjecture such models, and exemplify it by presenting an N=1 model conjectured to flow to an N=3 SCFT, deformed by a marginal deformation. We shall also discuss and exemplify how such proposals can be tested. December 4: Laura Donnay (TU Wien) Title: Asymptotic symmetries and celestial CFT Abstract: Universal relationships between asymptotic symmetries, QFT soft theorems, and low energy observables have reinvigorated attempts at flat space holography. In this talk, I will review recent advances in the celestial holography proposal, where the 4d S-matrix is reconsidered as a 2d correlator on the celestial sphere at null infinity. In this framework, asymptotic particle states are characterized by the point at which they enter or exit the celestial sphere as well as their SL(2,C) Lorentz quantum numbers: namely their conformal scaling dimension and spin instead of the energy and momentum. I will present a unified treatment of conformally soft Goldstone modes which arise when spin-one or spin-two conformal primary wavefunctions become pure gauge for certain integer values of the conformal dimension. December 2: Triangle Title: A novel twistor string theory (Kevin Costello - Perimeter Institute) Abstract: I will report on some work in progress on type I topological strings on twistor space, and their relationship with integrable but non-supersymmetric field theories in three and four dimensions. This is closely related to recent work of Bittleston and Skinner. Title: Numerical CY metrics from holomorphic networks (Michael Douglas - Simons Center) Abstract: We propose machine learning inspired methods for computing numerical Calabi-Yau metrics, and implement them using Tensorflow/Keras. We compare them with previous work, and find that they are far more accurate for manifolds with little or no symmetry. We also discuss issues such as optimization methods, overparameterization and computational complexity. Joint work with Subramanian Lakshminarasimhan and Yidi Qi. November 27: Papers We discussed: Massless particles in higher dimensions (Weinberg) - Ben, Sam, Dio How hot can de Sitter and black holes be? Akhmedov, Anempodistov, Bazarov, Diakonov, Moschella - Teresa Please post here papers/talks you will be interested in discussing this Friday: November 25: Riccardo Rattazzi Title: Multilegs, Superfluids and Semiclassics Abstract: Even in weakly coupled QFTs, perturbation theory breaks down when one considers amplitudes with a large number $n$ of legs. The series cleverly organizes as a double expansion in $g^2$ and $g^2n$. I show how the series in $g^2n$ can be fully captured by a semiclassical expansion around a non-trivial solution. Focussing on $U(1)$ symmetric $|\phi|^4$ theory in $4$ and $4-\epsilon$ dimension I derive explicit and consistent all order results for the anomalous dimension of the complex operator $\phi^n$. When restricting to the Wilson-Fisher fixed point and working on the cylinder, the dominant trajectory is seen to correspond to a superfluid phase for the conserved U(1). This creates a correspondence between, on one side, the spectrum of operators and fusion coefficients and on the other the spectrum of hydrodynamics modes and their interactions. The results also nicely match Monte Carlo simulations in 3D, compatibly with the stunt of taking $\epsilon=1$. November 20: Papers We discussed: Non-Lorentzian Avatars of (1,0) Theories (Tristan, Neil) The statistical mechanics of near-BPS black holes (Heydeman, Iliesiu, Turiaci, Zhao) - Alejandro November 18: Yang-Hui He (City) (Polygon) Title: Universes as Bigdata: from Geometry, to Physics, to Machine-Learning Abstract: We briefly overview how historically string theory led theoretical physics first to algebraic/differential geometry, and then to computational geometry, and now to data science. Using the Calabi-Yau landscape - accumulated by the collaboration of physicists, mathematicians and computer scientists over the last 4 decades - as a starting-point and concrete playground, we then launch to review our recent programme in machine-learning mathematical structures and address the tantalizing question of how AI helps doing mathematics, ranging from geometry, to representation theory, to combinatorics, to number theory. November 13: Gizem Sengor (CEICO, Institute of Physics of the Czech Academy of Sciences) Title: Some of the unintuitive features of de Sitter Abstract: We would like to study the late time properties of scalar fields on de Sitter, in terms of operators at the late time boundary. In this talk we will start by introducing these late time operators and recognizing them as unitary irreducible representations of SO(d+1,1), the symmetry group of d+1 dimensional de Sitter. We will discuss their inner products and two point functions where there exist subtle differences between handling light and heavy fields. November 11: Susanne Reffert (Bern U.) Title: The large charge expansion Abstract: In has become clear in recent years that working in sectors of large global charge of strongly coupled and otherwise inaccessible CFTs leads to important simplifications. It is indeed possible to formulate an effective action in which the large charge appears as a control parameter. In this talk, I will explain the basic notions of the large-charge expansion using the simple example of the O(2) model and then generalize to models with a richer structure which showcase other effects. November 6: Papers discussion We discussed: Conformal Regge Theory at Finite Boost Caron-Huot, Sandor - Andrea Constructing AdS_2 flow geometries - Damian/Dionysios November 4: Elli Pomoni (DESY) Title: Type-B Anomalies on the Higgs Branch Abstract: In this talk we will study type-B conformal anomalies associated with 1/2-BPS Coulomb-branch operators in 4D N=2 SCFTs. We will derive the conditions under which these anomalies can match across the conformal phase and the Higgs phase, and explicitly see them at work in concrete examples of both matching and non-matching. On the one hand matching leads to a new class of data on the Higgs branch of 4D N=2 SCFTs that are exactly computable. On the other, non-matching imposes novel restrictions on the holonomy of the conformal manifold. October 30: Papers discussion Simple holographic models of black hole evaporation youtube video (Akers,Engelhardt,Harlow) - Kevin From AdS to dS Exchanges: Spectral Representation, Mellin Amplitudes and Crossing, building on A Mellin Space Approach to Cosmological Correlators (Sleight, Taronna) - Teresa October 28: Clifford V. Johnson (USC) - Triangle Title: Non-perturbative physics for JT gravity and supergravity Abstract: Jackiw–Teitelboim (JT) gravity and supergravity theories are exciting solvable models that teach us about low temperature black hole dynamics, and aspects of quantum chaotic behaviour. They have been shown (by Saad, Shenker and Stanford, and by Stanford and Witten) to have double scaled random matrix model descriptions, capturing the (spacetime) topological perturbative expansion of the partition function. I will describe an alternative method for building the matrix model description, using techniques from minimal string theory. This method is particularly useful for supplying non-perturbative definitions of the physics. I show how this allows for the computation of key aspects of the physics at low energy and temperature. October 23: Tomas Reis (Geneva U.) Title: Renormalons from Resurgence Abstract: In this talk I will present the use of resurgence techniques in identifying non-perturbative physics and its relation to renormalons, in light of recent work of Marcos Mariño and me. This exploration has touched 1+1 QFTs and condensed matter systems, revealing that the presence of renormalons is more widespread than was previously expected. After introducing both ideas in the title, I will specialise in the case of the 1D fermion gas with a Dirac-delta attractive potential (Gaudin-Yang model). This case is a clean example of our approach and it unveils a curious interplay between resurgence, renormalons, and superconductivity. October 21: Thomas Mertens (Ghent U.) Title: Liouville and JT quantum gravity - holography and matrix models Abstract: In this talk, we will discuss recent progress in understanding quantum gravity amplitudes (partition function and boundary correlation functions) in Liouville gravity, and how they limit to Jackiw-Teitelboim (JT) correlators. We also discuss multiboundary and higher genus amplitudes. We focus on two main results: the Liouville gravity answers look like q-deformations of the JT answers, and Liouville gravity can be related to a 2d dilaton gravity with a sinh dilaton potential. We end with discussions on supersymmetric extensions and work in progress. Based largely on arXiv:2006.07072 and 2007.00998​. October 16: Papers discussion Gravitational Constrained Instantons (Cotler, Jensen) - Damian Conformal Gravity - Philip Mannheim (2012) - Chris October 14: Anatoly Dymarsky (U. of Kentucky) Title: Quantum codes, lattices, and CFTs. Abstract: There is a deep relation between classical error-correcting codes, Euclidean lattices, and chiral 2d CFTs. We show this relation extends to include quantum codes, Lorentzian lattices, and non-chiral CFTs. The relation to quantum codes provides a simple way to solve modular bootstrap constraints and identify interesting examples of conformal theories. In particular we construct many examples of physically distinct isospectral theories, examples of "would-be" CFT partition function — non-holomorphic functions satisfying all constraints of the modular bootstrap, yet not associated with any known CFT, and find theory with the maximal spectral gap among all Narain CFTs with the central charge c=4. At the level of code theories the problem of finding maximal spectral gap reduces to the problem of finding optimal code, leading to "baby bootstrap" program. We also discuss averaging over the ensemble of all CFTs associated with quantum codes, and its possible holographic interpretation. The talk is based on arXiv:2009.01236 and arXiv:2009.01244. October 9: Imtak Jeon Title: Quantum effects on Black hole Thermodynamics Abstract:Hello everyone! Recently, I suddenly received an offer for 5 year position as a research leader and now I am starting the position at APCTP in Korea. I appreciate faculties and colleagues in KCL for that they organised meetings on how to write proposal and how to do interview and also gave me feedback when I prepared those. Because of their support, I was able to succeed my interview. So I think it would be good if I give a talk that I presented for my interview. In this way, I can share how I did my interview wishing that this serves as a useful guide for students and postdocs. In this talk, I will sketch my motivation for Quantum black holes and their thermodynamics. And I will sketch my accomplishment and my future research directions. October 7: Gabor Sarosi (CERN) Title: The butterfly effect away from maximal chaos Abstract: A simple probe of chaos and operator growth in many-body quantum systems is the thermal out of time ordered (OTO) four point function. In a large class of local systems, the effects of chaos in this correlator build up exponentially fast inside a so called butterfly cone. I will discuss universal features of the spatiotemporal structure of this exponential growth in large N systems. In particular I will argue that there can be a smaller, “scramblon” cone inside the butterfly cone. Outside the scramblon cone, the growth of the OTO four point function is completely universal and saturates a chaos bound. I will explain the connection to conformal Regge theory, where crossing the scramblon cone is related to an exchange of dominance between the pomeron and the stress tensor. Finally, I will discuss a connection between chaos and energy transport, called the pole skipping phenomenon, which is a subtle effect in the thermal energy density retarded two point function at a special point in the complex frequency and momentum planes. I will present an improved understanding of this connection and test it in the large q limit of an SYK chain, where I determine both the Lyapunov growth of the OTO correlator and the energy density two point function exactly as a function of the coupling, interpolating between weekly coupled and maximally chaotic behaviour. October 2: Kévin Nguyen Title: The effective action of superrotation modes Abstract:Starting from an analysis of four-dimensional asymptotically flat gravity in first order formulation, we show that superrotation reparametrization modes are governed by an Alekseev-Shatashvili action on the celestial sphere. This two-dimensional conformal theory describes spontaneous symmetry breaking of Virasoro superrotations together with the explicit symmetry breaking of more general Diff(S2) superrotations. We arrive at this result by first reformulating the asymptotic field equations and symmetries of the radiative vacuum sector in terms of a Chern-Simons theory at null infinity, and subsequently performing a Hamiltonian reduction of this theory onto the celestial sphere. September 30: Teresa Bautista Solans Title: Lorentzian CFT 3-point functions and the ANEC Abstract: In CFT, the expressions for Euclidean 3-point functions in momentum space were fully obtained in recent years, but their Lorentzian counterparts have remained quite unknown. In this talk I will present the expression for the Lorentzian 3-point function of scalars, and further show a way to obtain tensorial ones. As I will argue, such momentum-space expressions simplify considerably the computation of the expectation values of the ANEC (Average Null Energy Condition) operator on the states used in the conformal colliders setting, whose positivity has been used to put interesting bounds on conformal anomalies. With the motivation of generalising these bounds and studying the implications of the ANEC for QFT, I will discuss perturbative corrections to the simplest ANEC expectation values in lambda-phi4 theory.