Tuesday 12:00-1:00pm in B44/1057 followed by informal discussion in B54/4001 (Ketley room).

Talks will also be broadcast on zoom.

Note: dates are subject to change, depending on whether topics run over into a second session.

Semester 1

21/09/21 - Online coffee break & organizational meeting

05/10/21 - Filip Landgren (1 pm)

12/10/21 - No journal club this week

19/10/21 - String seminar: Avia Raviv-Moshe (3 pm)

26/10/21 - Adam Chalabi (12:30 pm)

02/11/21 - String seminar: Yifan Wang, (12:30 pm)

09/11/21 - Jack Mitchell (12:30 pm)

16/11/21 - Arvind Shekar (12:30 pm)

23/11/21 - Davide Bufalini (12:30 pm)

30/11/21 - Mritunjay Verma (12:30 pm)

07/12/21 - String seminar: Debodirna Ghosh (12:30 pm)

[Winter break: 14 December 2020 - 15 January 2021]

Semester 2

11/01/22 - No Journal club this week

[Exam period: 13-24 January 2021]

01/02/22 - Callum Hunter (1pm)

08/02/22 - Ernesto Bianchi (12:00 pm)

15/02/22 - Enrico Parisini (12:00 pm)

22/02/22 - Alex Davey (12:00 pm)

01/03/22 - Daniel Hasenbichler (12:00 pm)

08/03/22 - Sami Rawash (12:00 pm)

15/03/22 - Organisational meeting & discussion of recent papers (12:00 pm)

22/03/22 - Luis Rocha (12:00 pm)

[Spring break: 26 March - 23 April 2021]

26/04/22 - No talk due to Eurostrings 2022

03/05/22 - Zezhuang Hao (12:00 pm)

10/05/22 - Davide Bufalini

17/05/22 - Kostas Skenderis

24/05/22 - String Seminar (see Strings calendar)

Topics: (reverse chronological order)

Kostas Skenderis : No Ensemble Averaging Below the Black Hole Threshold

I will review 2202.011372, No Ensemble Averaging Below the Black Hole Threshold, by Schlenker and Witten.

Davide Bufalini : Probing black hole microstates with the string worldsheet

One of the most important objectives of string theory is to account for the bulk entropy of the 3-charge black hole. Some microstates may not have a reliable supergravity description, and thus models that capture stringy physics may be essential. After reviewing the D1D5 system and its moduli space, I will present an exact worldsheet model [1] describing the propagation of a string in some non-BPS microstates, and show how to compute correlation functions of physical states in this heavy background. From the worldsheet, we obtain a new infinite family of new non-SUSY holographic Heavy-Light correlators involving an arbitrary number of light states in the chiral multiplet, with arbitrary conformal dimension and R-charge. These correlation functions appear to be accidentally protected, and perfectly match very few previously known cases computed at the orbifold point of the D1D5 CFT. We briefly comment on relations with Little String Theory, tensionless strings in AdS3 and, potentially, flat holography.

This talk is based on works done in collaboration with Sergio Iguri, Nicolas Kovensky and David Turton [2,3,4].

[1] Martinec, Massai - arXiv:1705.10844

[2] Bufalini, Iguri, Kovensky, Turton - arXiv:2105.02255

[3] Bufalini, Iguri, Kovensky, Turton - arXiv:2203.13828

[4] Bufalini, Iguri, Kovensky, Turton – to appear (soon)

Zezhuang Hao: Emergent Times in Holographic Duality

The duality between a two-side eternal black hole and a copy of non-interacting CFTs has been brought up over two decades while the machinery behind this is still not as clear as the duality with single boundary. If the boundary CFTs can talk with each other? How can we construct the map between the bulk and boundary operators? In this talk, we will make such problems sharper from the algebraic point of view and shape them into a thought experiment. After a brief discussion of the thought experiment, we will go into a light introduction of the classification of different types of von Neumann algebra and the Tomita-Takesaki theory. Then we will present some recent results in Leutheusser, Liu’s emergent time paper and Witten’s gravity as cross product paper, which throw new lights on the understating of time, horizon and singularities in the context of algebraic QFT.

Luis Rocha: Orbifold compactification of the pure spinor formalism

After a brief review of the pure spinor formalism for the superstring, I will discuss what is required to consistently define the theory on a six-dimensional orbifold. As an application, I will describe how topological amplitudes of type II superstrings in these backgrounds are obtained at arbitrary genus, and also explain what difficulties are expected when extending the discussion to non-topological amplitudes.

Sami Rawash: Shockwaves in black hole microstate geometries

This talk is based on 2112.08378. After reviewing the construction of two-charge black hole microstates in five dimensions and their CFT duals, I will discuss the backreaction of a shockwave on the simplest of these backgrounds (the circular supertube) and I will identify a family of CFT states dual to this configuration. I will then discuss how to use a spectral flow transformation to construct the first family of asymptotically-flat supersymmetric three-charge microstate solutions that contain a shockwave.

Daniel Hasenbichler: Asymptotics in Relativistic Hydrodynamics

I give a short introduction into the MIS model of hydrodynamics. For a toy model that behaves qualitatively like an expanding N=4 SYM plasma, I discuss the temperature and pressure anisotropy evolution in proper time and study the asymptotic late-time expansions of the solutions to a non-linear first order ODE. The late-time behaviour is given as a one-parameter transseries and must be matched to the physical solution at early times. I present different ways of doing the matching and elaborate on the method of Transasymptotic summation. I discuss how global properties of the solution can be deduced from the local behaviour at infinity.

Alex Davey: Strong Cosmic Censorship in BTZ

Unlike other asymptotically AdS black holes, BTZ violates the Strong Cosmic Censorship conjecture. This is a surprising result both from the perspective of the scalar field on a fixed BTZ background, and the renormalised stress-energy tensor in the Hartle-Hawking state. I will discuss these results, and then review some proposals in the literature for how one might understand this violation holographically.

Enrico Parisini: Spatial Infinity and Asymptotically Flat Holography

Asymptotically flat spaces pose a number of problems for the definition of a holographic dual theory, especially when considering more general bulks than Minkowski. In this talk I will illustrate how using a particular hyperbolic foliation close to spatial infinity (the Beig-Schmidt gauge) may shed light on some of these issues. As a main example, I will show in detail how from this perspective the so-called antipodal matching conditions between past and future null infinity naturally arise from Einstein's equations. I will also touch upon other problems of flat holography such as superrotations and time evolution.

Ernesto Bianchi: Quantum Holographic Renormalization

Known are the conformal correlators dual to scalar theories on AdS through the classical approximation of the AdS/CFT correspondence, confirming the conjecture at this level. In this talk we will explore its consequences beyond this approximation, considering the quantum corrections of the correspondence for the particular case of a scalar Phi^4 theory on a fixed AdS background. This will involve generalizing the known holographic renormalization procedure to its quantum version. At the end, this process will show in a clear way the effect of these loop corrections to the dual correlators through the main role played by the infrared and ultraviolet divergences, which is briefly summarized in: the quantum corrections produce anomalies in the resulting CFT in the form of an anomalous scaling dimension, confirming the validity of the conjecture beyond tree-level computations. All this study is carried out throughout this talk for arbitrary dimension and parameters of the theory in the bulk.

Callum Hunter: Effective Field Theories and their UV Completions

In this talk I will present the general introduction to so-called positivity bound and briefly discuss the formulation that leads to constraints on the coefficients of bottom-up effective field theories resulting from general S-Matrix constraints. Using the scalar 2->2 scattering amplitudes of bottom up Scalar EFTs it is possible to constrain the Scalar EFT’s constraints such that the UV completion obeys the S-Matrix constraints: Lorentz Invariance, Analyticity, Causality and Crossing Symmetry. These bounds can then be extended to spin-1 and -2 field theories by adopting the transversity formalism for scattering amplitudes, opening up the possibility of constraining EFTs of gravity. I will then spend the latter half of the talk exploring applications of such EFT bounds including a discussion of Black Holes in the Einstein-Maxwell EFT, constraints on cosmological Horndeski theory and the massive Galileon model – a particularly important model in dRGT gravity.

Debodirna Ghosh: Supertranslations at timelike infinity

In this talk, I will propose a definition of asymptotic flatness at timelike infinity in four spacetime dimensions. I will also present a detailed study of the asymptotic equations of motion and the action of supertranslations on asymptotic fields. I will then show that the Lee-Wald symplectic form does not get contributions from future timelike infinity with appropriate boundary conditions. As a result, the “future charges” can be computed on any two-dimensional surface surrounding the sources at timelike infinity. Finally, I will present expressions for supertranslation and Lorentz charges.

Mritunjay Verma: Higher spin fields and gyromagnetic couplings

We analyze the gyromagnetic couplings in the Bosonic, Heterotic, and Type II string theories compactified on a generic torus having constant moduli. By computing the hamiltonian giving the interaction between massive string excitations and U(1) gauge fields arising from the graviton and Kalb-Ramond field upon compactification, we derive a general formula for such couplings that turns out to be universal in all these theories. For a generic mixed symmetry state, there is one gyromagnetic coupling associated with each row of the corresponding Young Tableau diagram. For all the states having zero Kaluza Klein or Winding charges, the value of g turns out to be 1. We explicitly consider totally symmetric and mixed symmetry states (having two rows in the Young diagram) associated with the first Regge-trajectory and obtain their corresponding g value.

Davide Bufalini: Black Hole = String

This talk focuses on the recent paper by Chen, Maldacena and Witten [1]. I first review previous ideas on the black hole/string transition and arguments showing connections between an excited fundamental string and black holes are provided. I then review the Horowitz-Polchinski (HP) solution which will enable us to discuss the paper [1], where the authors provide a sigma model analysis pointing towards the possible connection between the HP solution and the black hole as (classical) string theory solutions.

Lastly, the solution generating technique used in [1] gives a charged version of the HP solution with fuzzball-like features: connections with fuzzballs are discussed.

[1] arXiv:2109.08563

Arvind Shekar: Circuit complexity in interacting QFTs and RG flows

There has been exciting progress in understanding the connection between entanglement and geometry. However, there are explicit examples that demonstrate entanglement entropy is not enough to capture all the information in the bulk geometry. A natural question as to what quantities in the field theory bridge to this bulk physics (not captured by entanglement entropy) leads us to expand our information theory tools to also include ‘Complexity’. Being able to compute and have an explicit expression for the complexity of boundary field theories will put us in a better position to make comparisons with bulk quantities and maybe even establish a concrete translation to new observables in the bulk (as was found for holographic entanglement entropy). Although several attempts to understand Circuit complexity for QFTs have been made, it had only been computed for free field theories. We take a step towards understanding Circuit Complexity in certain interacting scalar quantum field theories, mainly focusing on the \phi^4 theory and O(N) scalar field theory. We work out the circuit complexity for evolving from a nearly Gaussian unentangled reference state to the entangled ground state of the theory. Our approach uses Nielsen’s geometric method, which translates into working out the geodesic equation arising from a certain cost functional and we present an expression for complexity by doing the required lattice sums analytically. Our method enables a study of circuit complexity in the epsilon expansion for the Wilson-Fisher fixed point. We find that with increasing dimensionality the circuit depth increases in the presence of the \phi^4 interaction eventually causing the perturbative calculation to breakdown. We discuss how circuit complexity relates with the renormalization group.

Jack Mitchell: Domain Wall Fermions on the Brane

In this talk domain wall fermions and their condensation in the D3/probe D7 system will be presented. A spatially dependent mass term for the N = 2 hypermultiplet can be arranged to isolate distinct two component fermions on two 2+1 dimensional domain walls. Here the system shows condensation/mass generation analogous to the D3/probe D5 anti-D5 system. The chiral condensate and pion mass can be directly computed on the domain wall. It is shown that chiral symmetry breaking can be induced by a dilaton flow that causes the core of the space to become sufficiently repulsive. Several such flows are detailed, and a domain wall pair are embedded into a confining background in a top down model of ''Domain Wall AdS/QCD'' from which observables are calculated. Improving the model by including the effects of higher dimension operators is also discussed.

Adam Chalabi: Central Charges of Conformal Defects

In this talk I will explain how conformal defects can be characterised by their contributions to the trace anomaly of the stress tensor. I will give examples of 2d conformal defects where the trace anomaly can be computed exactly, without requiring any approximations or limits. I will discuss a simple example in free theories based on [2104.01220]. In another set of examples based on [2003.02857], I will explain how supersymmetry allows for straightforward computation of defect central charges in interacting conformal field theories. Finally, I will discuss a paper in preparation on 4d conformal defects in which we determine the general form of the defect trace anomaly and study various examples.

Filip Landgren: Escaping the Swampland with braneworlds: an explicit 3d de Sitter cosmology in string theory

Motivated by the lack of consensus on whether or not de Sitter (dS) lies in the Swampland, we use a recently developed braneworld construction and known Anti-de Sitter (AdS) vacua to compute an explicit effective dS cosmology in three dimensions. We consider a non-perturbative AdS4 vacuum decaying to another lower AdS4 vacuum via bubble-nucleation. We also consider the more speculative case where a dS4 decay to a Minkowski4. The Israel junction conditions are solved across the bubble and we obtain the Friedmann equations from which the cosmological constants can be read off, in the respective cases. The cosmological constants are computed in a flux background yielding small positive values admitting a dS cosmology.

Reference: https://arxiv.org/abs/2109.06801