fall2021
KCL Strings Journal Club Fall 2021

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Fall 2021 Journal Club Here's a summary of all the activity of our Journal Club during the Fall 2021 term.

Summary

Wednesday December 15, @ 13:15h: Alejandra Castro

Room: Online. Screening in K0.20

Title: Near AdS_2 spectroscopy

Abstract: In this talk I will describe holographic properties of near-AdS_2 spacetimes that arise within spherically symmetric configurations of N=2 4D supergravity, for both gauged and ungauged theories. These theories pose a rich space of AdS_2xS^2 backgrounds, and their responses in the near-AdS_2 region are not universal. I will show that the spectrum of operators dual to the matter fields, and their cubic interactions, are sensitive to properties of the background and the theory it is embedded in. The properties that have the most striking effect are whether the background is BPS or non-BPS, and if the theory is gauged or ungauged. The resulting differences will have an imprint on the quantum nature of the microstates of near-extremal black holes, reflecting that not all extremal black holes respond equally when kicked away from extremality.

Wednesday December 8, @ 13:15h: Emily Nardoni

Room: Online. Screening in K0.20

Title: Holographic Duals of Argyres-Douglas Theories

Abstract: The strongly coupled Argyres-Douglas field theories have particular significance among four-dimensional N=2 SCFTs. In this talk, we describe new AdS5 solutions in 11d supergravity and identify them as the gravity duals of a large class of Argyres-Douglas theories, engineered via a stack of M5-branes wrapping a sphere. A notable feature of the gravity solutions is an internal M5-brane source, which is dual to an irregular puncture on the sphere. We explain how the holographic data (including central charges) match the data of the dual Argyres-Douglas field theories

Wednesday November 24, @ 13:15h: Matthijs Hogervorst

Room: K0.20

Title: Conformal bootstrap meets cosmology

Abstract: Local observables in a de Sitter universe become conformal, if you wait long enough. Indeed, one can study the imprints of inflation by looking at conformal correlations in the sky. There’s an ongoing effort in the cosmology community to understand these late-time correlators from first principles, without invoking a specific Lagrangian.
In this talk, I will discuss the late-time CFT living in de Sitter through the lens of a quantum field theorist. The CFT in question shares many features with its counterparts in flat space or AdS, but differs in crucial aspects: in particular, it can have complex scaling dimensions and correlation functions. I will nevertheless argue that de Sitter CFTs have good unitarity properties and can be constrained via conformal bootstrap equations. This observation should open up a new way to constrain cosmological correlation functions.

Wednesday November 17, @ 17:00h: Maxim Metlitski

Screening room: K6.63

Title: Boundary criticality of the O(N) model in d = 3 critically revisited

Abstract: It is known that the classical O(N) model in dimension d > 3 at its bulk critical point admits three boundary universality classes: the ordinary, the extraordinary and the special. The extraordinary fixed point corresponds to the bulk transition occurring in the presence of an ordered boundary, while the special fixed point corresponds to a boundary phase transition between the ordinary and the extra-ordinary classes. While the ordinary fixed point survives in d = 3, it is less clear what happens to the extraordinary and special fixed points when d = 3 and N is greater or equal to 2. I'll show that formally treating N as a continuous parameter, there exists a finite range 2 < N < N_c where the extra-ordinary universality class survives, albeit in a modified form: the long-range boundary order is lost, instead, the order parameter correlation function decays as a power of log r. I'll discuss recent Monte-Carlo simulations and numerical bootstrap results that confirm the above picture and indicate that the critical value N_c > 3.
Based on arXiv:2009.05119, 2111.03613, 2111.03071

Wednesday November 10, @ 13:15h: Joaquin Turiaci

Room: Online (Screening in k0.20)

Title: Phase transitions for deformations of JT supergravity and matrix models

Abstract: We analyze black holes in deformations of Jackiw-Teitelboim (JT) supergravity by adding a gas of defects, equivalent to changing the dilaton potential. For some range of deformations, the black hole density of states extracted from the gravitational path integral becomes negative, yielding an ill-defined sum over topologies. To solve this problem, we use an equivalent matrix model description and show the negative spectrum is resolved via a phase transition analogous to the Gross-Witten transition. The matrix model contains a rich and novel phase structure that we explore in detail, using both perturbative and non-perturbative techniques.

Monday November 08, @ 15:15h: Claire Zukowski

Room: Norfolk Building G.01

Title: Berry Phases and Complexity as Probes of Bulk Geometry

Abstract:I will describe two new quantum information theoretic probes of bulk geometry that access information inaccessible to spacelike geodesics. The first arises from considering a parallel transport process of modular Hamiltonians on the boundary under a change of state. I will show that the Berry curvature for this process computes the entanglement wedge symplectic form associated to a family of Euclidean cosmic brane solutions. Next, I will derive the circuit complexity for conformal field theory in arbitrary dimensions. I will show that circuits are dual to timelike geodesics in the bulk, and that the complexity metric admits a simple bulk geometric description in terms of distances between geodesics. In either case, these quantities are governed by the geometry of coadjoint orbits, which are special symplectic manifolds arising from group theory. The state-changing modular Berry transport process naturally describes the geometry of new, Virasoro-like coadjoint orbits that extend beyond the current classification. The complexity metric describes the geometry of a particular coadjoint orbit of the conformal group in arbitrary dimensions.

Wednesday November 03, @ 13.15h: Dalimil Mazac

Room: Online

Title: AdS Bulk Locality from Sharp CFT Bounds

Abstract: It has been a long-standing conjecture that any CFT with a large central charge and a large gap M in the spectrum of single-trace operators must be dual to a local effective field theory in AdS. In my talk, I will discuss a proof of a sharp form of this conjecture. In particular, I will explain how to derive numerical bounds on bulk Wilson coefficients in terms of M using the conformal bootstrap. The bounds exhibit scaling in M expected from dimensional analysis in the bulk. The main technical tools are dispersive CFT sum rules. These sum rules provide a dictionary between CFT dispersion relations and S-matrix dispersion relations in appropriate limits. This dictionary allows one to apply recently-developed flat-space methods to construct positive CFT functionals.

Tuesday November 02, @ 13.00h: Jennifer Lin

Room: K0.16

Title: A new look at the gravitational entropy formula

Abstract: The Ryu-Takayanagi formula and its generalizations have led to a surprising amount of progress in our understanding of quantum gravity in the last fifteen years, culminating in the recent derivation of the Page curve in toy models of evaporating black holes. However, we still don’t understand why these formulas are true from a canonical point of view. In this talk, I will attempt to make progress on this problem by developing an analogy between gravitational entropy formulas in low-dimensional examples of holography and similar-looking formulas that have appeared in the study of entanglement entropy in emergent gauge theories. This talk will be based on 1807.06575, 2107.11872, and 2107.12634.

Triangle seminar: Shota Komatsu

Wednesday October 20, @ 15.15h: Avia Raviv-Moshe

Room: Online

Title: Renormalization Group Flows on Line Defects

Abstract: In this talk, we will consider line defects in d-dimensional CFTs. The ambient CFT places nontrivial constraints on renormalization group flows on such line defects. We will see that the flow on line defects is consequently irreversible and furthermore a canonical decreasing entropy function exists. This construction generalizes the g theorem to line defects in arbitrary dimensions. We will demonstrate this generalization in some concrete examples, including a flow between Wilson loops in 4 dimensions, and an O(3) bosonic theory coupled to impurities with large isospin.

Wednesday October 13, @ 13.15h: Paul Ryan

Room: K0.20

Title: Separation of variables and correlation functions in high-rank integrable systems

Abstract: The spectral problem for N=4 Super Yang-Mills can be formulated as a set of quantisation conditions on a handful of functions called Q-functions. Recent analysis suggests that the Q-functions can be used as simple building blocks for 3-point correlation functions. This strongly resembles the situation in integrable spin chains where the wave functions factorise into a simple product of Q-functions in a special basis called Sklyanin’s separation of variables (SoV) basis which is one of the most powerful approaches for solving integrable systems. Unfortunately this framework has only been developed for the simplest integrable spin chains with sl(2) symmetry, far from the psu(2,2|4) needed to describe N=4 SYM. In this talk I will review recent advances in developing the SoV approach for higher rank integrable spin chains. I will explain how to construct the SoV basis in a systematic fashion and how it links to the representation theory of the system. Next, I will discuss a new approach for obtaining the measure in separated variables based on the famous Baxter TQ equation and how the approach naturally provides a large family of correlation functions as very simple determinants in Q-functions. I will briefly discuss how the approach can be applied directly to certain 4d QFTs, in particular the fishnet cousin of N=4 SYM.

Wednesday October 6, @ 13:15: Pietro Benetti Genolini

**Room: K0.20

Title: Instantons, symmetries and anomalies in five dimensions

Abstract: Five-dimensional non-abelian gauge theories have a U(1) global symmetry associated with instantonic particles. I will describe a mixed 't Hooft anomaly between this and other global symmetries of the theory, namely the one-form center symmetry or ordinary flavor symmetry for theories with fundamental matter. I will then apply these results to supersymmetric gauge theories, analysing the symmetry enhancement patterns occurring at their conjectured RG fixed points.

Wednesday September 29, @ 13:15: Marco Serone

**Room: online

Title: In search of fixed points in non-abelian gauge theories using perturbation theory

Abstract: Four-dimensional gauge theories can flow in the IR to non-trivial CFTs.
By employing Borel resummation techniques both to the ordinary perturbative series and to the Banks-Zaks conformal expansion, we first analyze the conformal window of QCD and find substantial evidence that QCD with n_f=12 flavours flows in the IR to a CFT.
We then study UV fixed points for SU(n_c) gauge theories with fundamental fermion matter in 4+2epsilon dimensions. Using resummation techniques similar to those used in the 4d QCD case, we provide evidence for the existence of non-supersymmetric CFTs in d=5 space-time dimensions in a certain range of colors and flavours.

Wednesday September 22, @ 13:45: Luca Iliesiu (Stanford U.)

**Room K2.31

Title: The volume of the black hole interior at late times

Abstract: Understanding the fate of semi-classical black hole solutions at very late times is one of the most important open questions in quantum gravity. In this paper, we provide a path integral definition of the volume of the black hole interior and study it at arbitrarily late times for black holes in various models of two-dimensional gravity. Because of a novel universal cancellation between the contributions of the semi-classical black hole spectrum and some of its non-perturbative corrections, we find that, after a linear growth at early times, the length of the interior saturates at a time, and towards a value, that is exponentially large in the entropy of the black hole. This provides a non-perturbative confirmation of the complexity equals volume proposal since complexity is also expected to plateau at the same value and at the same time.

Monday September 13, @ 15h: Mathew Bullimore (Durham U.)

**Room S0.03

Title: Towards a mathematical definition of the 3d superconformal index

Abstract: The aim of this talk is to give a mathematical definition of the superconformal index of 3d supersymmetric gauge theories. This can be computed exactly using supersymmetric localisation, leading to an explicit contour integral formula involving infinite q-Pochammer symbols. I will explain how this may be understood as the Witten index of a supersymmetric quantum mechanics, or index of a twisted Dirac operator on a certain infinite-dimensional space closely related to one introduced by Braverman-Finkelberg-Nakajima.

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