STAG Research Centre, University of Southampton, Highfield Campus

Time: Wednesday 1:00pm (PhD student session), 1:15-2:15pm (main seminar), unless otherwise stated.

Place: Semester 1: Building 54, Room 8031,

Semester 2: Building 54, Room 10031, unless otherwise stated.

Semester 1

24/09/19 - Antonin Portelli (Edinburgh) - *note Tuesday* - Time 1:00/1:15pm, Room 54/7033 (7C).

02/10/19 - Edoardo Lauria (Durham)

09/10/19 - Masanori Hanada (Southampton)

15/10/19 - Perseas Christodoulidis (Groningen) *note Tuesday* - Time 12:00/12:15pm, Room 13/3021

16/10/19 - Tarek Anous (Amsterdam)

23/10/19 - Martin Wolf (Surrey)

30/10/19 - Congkao Wen (QMUL)

06/11/19 - Ben Withers (Southampton)

14/11/19 - Nathan Seiberg (IAS) - String-Gravity seminar - *note Thursday* - 12:00pm, Room 02A/2077 (L/T J)

19/11/19 - Bidisha Chakrabarty (ICTS-TIFR Bangalore) - *note Tuesday* - Time 12:00/12:15pm, Room 13/3021

20/11/19 - Monica Guica (Saclay)

27/11/19 - David Schaich (Liverpool)

03/12/19 - Fabrizio del Monte (SISSA) - *note Tuesday* - Time 12:00/12:15pm, Room 13/3021

04/12/19 - Lorenzo Bianchi (QMUL)

10/12/19 - Josh Kirklin (Cambridge) - *note Tuesday* - Time 12:00/12:15pm, Room 13/3021

11/12/19 - Enrico Pajer (Cambridge)

- Winter break -

Semester 2

29/01/20 - Nicolas Kovensky (Southampton)

05/02/20 - José Manuel Penín (Southampton)

12/02/20 - Shira Chapman (Amsterdam)

19/02/20 - Masazumi Honda (Cambridge) - seminar + 3 informal lectures on 17/02 and 18/02, see below

26/02/20 - Mike Blake (Bristol) - *note unusual room/time* - 3:30/3:45pm, Room 54/7033

04/03/20 - Radu Tatar (Liverpool)

11/03/20 - Bogdan Stefanski *postponed*

18/03/20 - Manuela Kulaxizi *postponed* - replaced by a second session of the journal club

planned for 17/03/20, led by Sami Rawash.

25/03/20 - Alex Belin (CERN) *postponed*

- Spring break -

22/04/20 - Marika Taylor (Southampton) - *note unusual time* - 2:00pm - introductory lecture (second session of journal club planned for 21/04/20)

29/04/20 - Alex Belin (CERN)

(previous speaker Alice Bernamonti (Florence) postponed)

05/05/20 - Lampros Lamprou *postponed*

06/05/20 - Davide Bufalini (Southampton)

(previous speaker Stefano Cremonesi (Durham) postponed)

13/05/20 - Alessandro Torrielli (Surrey)

20/05/20 - Juan Pedraza (UCL) - *note unusual time* - 2:00/2:15pm

27/05/20 - Manuela Kulaxizi (Trinity College Dublin)

Titles & Abstracts (reverse chronological order)

Manuela Kulaxizi (Trinity College Dublin): The stress tensor sector of CFT correlation functions and black holes

We will study the stress tensor sector of CFT four point functions in the large central charge limit, by considering the correlation of two pairwise identical operators: two heavy (whose conformal dimension scales with c) and two light ones (whose conformal dimension is of order unity). We will find some interesting structures governing both the light-cone and the subheading to light-cone regimes of the stress-tensor sector in the <HHLL> correlator.

Juan Pedraza (UCL): Local quenches, bulk entanglement entropy and a unitary Page curve

Quantum corrections to the entanglement entropy of matter fields interacting with dynamical gravity have proven to be very important in the study of the black hole information problem. We consider a one-particle excited state of a massive scalar field infalling in a pure AdS3 geometry and compute these corrections for bulk subregions anchored on the AdS boundary. In the dual CFT2, the state is given by the insertion of a local primary operator and its evolution thereafter. We calculate the area and bulk entanglement entropy corrections at order O(N^0), both in AdS and its CFT dual. The two calculations match, thus providing a non-trivial check of the FLM formula in a dynamical setting. Further, we observe that the bulk entanglement entropy follows a Page curve. We explain the precise sense in which our setup can be interpreted as a simple model of black hole evaporation and comment on the implications for the information problem.

Alessandro Torrielli (Surrey): Massless scattering in lower-dimensional AdS/CFT

In recent years the framework of AdS/CFT integrability has seen one particular paradigm change due to the appearance of massless modes in the associated scattering problem. We shall briefly recap, following Zamolodchikov, ideas on massless integrable scattering. We shall then translate this scenario to the case of AdS3/CFT2, and demonstrate the appearance of a 2D relativistic critical theory in the BMN limit of the integrable scattering problem. We will study the thermodynamic Bethe Ansatz (TBA) of this theory, and reflect on recent work such as the one by Abbott and Aniceto which might help formulating a TBA for the full theory. This talk is primarily based on work with R. Borsato, A. Fontanella, O. Ohlsson Sax, A. Sfondrini, B. Stefanski and J. Stroemwall.

Davide Bufalini (Soton): The Wess-Zumino-Witten model. A geometric introduction

I will deliver a pedagogical introduction to the Wess-Zumino-Witten model, focusing mainly on geometric aspects of it. We will start by recalling what a non-linear sigma model is and then we will understand the properties and structure behind the WZW model by means of basic differential-geometric concepts that will be very natural in this context. We will also discuss the idea behind the gauging of the WZW model and start to understand its importance for the Microstate Geometries programme.

Alexandre Belin (CERN): New holographic CFTs: finding the needle in the haystack

In this talk, I will give evidence for a new infinite family of holographic CFTs in d=2. These are chaotic CFTs with a large central charge and a large gap to higher spin operators. The theories are obtained by taking the symmetric orbifold of any N=(2,2) minimal model and deforming away from the orbifold point. I will show that exactly marginal operators that enable the turning on of this coupling always exist, and that the BPS spectrum of these theories exhibits a slow growth compatible with a supergravity spectrum in AdS_3. Based on the paper https://arxiv.org/abs/2002.07819.

Bogdan Stefanski (City, London): Integrability in AdS3/CFT2 and beyond

I will begin by reviewing integrability in AdS3/CFT2, focusing on: (i) the protected spectrum, (ii) integrability of the backgrounds supported by NSNS flux, and (iii) the massless TBA. Time permitting, I will show how integrable string theories arise from holomorphic Chern-Simons theory.

Radu Tatar (Liverpool): de Sitter in the String Landscape

I argue that four-dimensional effective field theory descriptions with de Sitter isometries could be allowed in the presence of time-dependent internal degrees of freedom in type IIB string landscape. Moduli stabilization and time-independent Newton constants are possible in such backgrounds. However once the time-dependences are switched off, there appear no possibilities of effective field theory descriptions.

Mike Blake (Bristol): Hydrodynamics and Chaos in Quantum Matter

In this talk I will discuss how studies of holographic theories have led to the discovery of new connections between hydrodynamics and chaos in quantum many-body systems. In the first half of this talk I will describe evidence for a surprising relationship between the energy diffusion constant of strongly interacting theories and the butterfly velocity, which describes the speed at which many-body chaos propagates. In the second half of the talk I will discuss more recent developments that have led to a more precise relation between the energy density Green’s function of maximally chaotic systems and out-of-time ordered correlation functions. This new relation, known as `pole-skipping’, implies that chaos provides non-perturbative constraints on the dispersion relations of hydrodynamic collective modes in such systems.

Masazumi Honda (Cambridge): Evidence for a Non-Supersymmetric 5d CFT from Deformations of 5d SU(2) SYM

We study supersymmetry breaking deformations of the N=1 5d fixed point known as E1, the UV completion of SU(2) super-Yang-Mills. The phases of the non-supersymmetric theory can be characterized by Chern-Simons terms involving background U(1) gauge fields, allowing us to identify a phase transition at strong coupling. We propose that this may signify the emergence of a non-trivial, non-supersymmetric CFT in d=4+1 dimensions. This talk is based on a collaboration with Pietro Benetti Genolini, Hee-Cheol Kim, David Tong and Cumrun Vafa.

Masazumi Honda (Cambridge): Introduction to Quantum Computation for Particle Physicists (3 lectures) [slides]

I will talk on potential applications of quantum computation to quantum field theory in order to prepare for coming era of "quantum supremacy". I start with some basic concepts of quantum computation such as qubits, gates and measurement. Then I explain how to implement quantum simulation of a simple spin system and how to rewrite quantum field theories in terms of qubits. I also discuss quantum algorithms to compute various observables in quantum field theory. This talk includes some demonstrations of running quantum algorithm by using the IBM Quantum Experience cloud platform.

Shira Chapman (Amsterdam): On the Complexity of Black Holes

In the last few years, quantum computational complexity from quantum information theory has become a thriving topic of research in the high-energy community due to its relationship to black holes. This relation stems from the AdS/CFT correspondence, which relates the properties of quantum field theories to those of gravitational systems. I will describe these developments in detail. In particular, I will discuss our progress towards understanding complexity for quantum field theory states and the study of the relevant dual gravitational observables. I will explain the implications of these ideas on studying the interior of black holes.

3-D5 geometries and its propertiesD3-D5 geometries and its properties

José Manuel Penín (Southampton): Backreacted D3-D5 geometries and its properties

Abstract: We study the geometries originated by adding D5-branes intersecting D3-branes along a 2+1 dimensional subspace, considering the backreaction of the D5s. We construct the configuration realizing the addition of both massless and massive flavors at zero T and generalize the results of massless fundamentals to the case of having a non zero T field theory dual. We explore the properties of these backgrounds by computing several observables, such as Wilson loops, entanglement entropies or studying the hydrodynamics. By considering a more generic brane profile which vanishes in the UV we can also construct solutions with two possible behaviours in the IR: an isotropic fixed point or a geometry with anisotropic Lifshitz like scaling symmetry.

Nicolas Kovensky (Southampton): Kähler moduli stabilization from ten dimensions [slides]

Abstract: We describe the back-reaction of gaugino condensates in supersymmetric AdS4 Type II String Theory

compactifications with fluxes by using generalized complex geometry. We describe the modification of the

ten-dimensional supersymmetry equations and show that the cosmological constant prevents the cycle wrapped

by the branes with gaugino condensation from shrinking to zero size. Thus, unlike in ordinary geometric transitions

in flat space, the volume of this cycle remains finite. For D7 branes with gaugino condensation, this gives a

ten-dimensional account of Kähler moduli stabilization. Furthermore, by matching the ten-dimensional supergravity

solutions near and far from the cycle wrapped by the D7 branes, we find a relation between the size of this cycle

and the cosmological constant. This relation is almost exactly the one found by KKLT using 4d EFT.

Enrico Pajer (Cambridge): From Symmetries to Cosmological Observables

Abstract: According to our standard model of cosmology, inflation gives us the privilege to observationally test perturbative quantum field theory in curved spacetime with quantised dynamical gravity. The natural observables are correlation functions of metric perturbations. In this talk I will discuss how symmetries of the laws of physics constrain the shape of correlators. For non-linearly realised symmetries, examples include adiabatic modes in single field inflation in both flat and curved universes, and shift symmetric theories. Then I'll present two no-go theorems on the possible linearly realised symmetries, a cosmological analog of the Coleman-Mandula theorem. Assuming scale invariance, homogeneity and isotropy, the largest amount of symmetries of a generic scalar correlator is the de Sitter group, equivalently the conformal group in three Euclidean dimensions. Moreover, assuming a single-clock evolution, the correlators of curvature perturbation can be de Sitter invariant only if the theory is free.

Josh Kirklin (Cambridge): Holographic Uhlmann Holonomy and the Entanglement Wedge Symplectic Form

Abstract: Subregion duality is an idea in holography which states that every subregion of the boundary theory has a corresponding subregion in the bulk theory, called the 'entanglement wedge', to which it is dual. In the classical limit of the gravity theory, we expect the fields in the entanglement wedge to permit a Hamiltonian description involving a phase space and Poisson brackets. In this talk, I will describe how this phase space arises from the point of view of the boundary theory. In particular, I will explain how it emerges from measurements of a certain quantum information-theoretic quantity known as the 'Uhlmann phase', in the boundary subregion.

Lorenzo Bianchi (QMUL): "Symmetries and surfaces"

Abstract: In this talk I will give an overview of recent advances in the study of (super)conformal defects. First, I will discuss some general aspects of the subject introducing a few physically relevant examples of defects. Then, I will focus on the case of surface defects in four-dimensional superconformal theories and introduce two complementary approaches for the exact computation of defect correlators.

Fabrizio del Monte (SISSA): Class S theories and isomonodromic deformations on the torus

In the last few years there have been many new results connecting (linear quiver) N=2 class S theories, and the topological strings that engineeer them, to the theory of isomonodromic deformations on the sphere and their q-deformations. These gauge theories are constructed by compactifying the 6d N=(0,2) SCFT on a punctured Riemann Sphere, whose moduli, which are the marginal deformations of the gauge theory, are the times of the isomonodromic flows.

The aim of this talk is to show how this connection can be extended beyond the case of genus zero, for more general (asymptotically superconformal) class S theories. We will discuss in detail the case of circular quiver gauge theories, that are obtained from the N=(0,2) SCFT on punctured tori, and see how the genus one case displays new qualitative features that are absent on the sphere, due to the possibility of various inequivalent vector bundles, and how this actually provides new interesting relations satisfied by the gauge theory partition function.

David Schaich (Liverpool): Maximally supersymmetric Yang-Mills on the lattice

Lattice field theory provides a non-perturbative regularization suitable for strongly interacting systems, which has proven crucial to the study of quantum chromodynamics among many other theories. Lattice investigations of supersymmetric field theories have a long history but often struggle due to the interplay of supersymmetry with the lattice discretization of space-time. I will discuss a way around these difficulties for maximally supersymmetric Yang--Mills theories, which are a cornerstone of holographic duality. After reviewing some highlights of the lattice formulation, I will present a selection of results from recent and ongoing numerical studies, including tests of holography.

Monica Guica (Saclay): T-Tbar and the mirage of a bulk cutoff [slides]

This talk will be based on the paper https://arxiv.org/abs/1906.11251. We use the variational principle approach to derive the large N holographic dictionary for two-dimensional T-Tbar-deformed CFTs, for both signs of the deformation parameter. The resulting dual gravitational theory has mixed boundary conditions for the non-dynamical graviton; the boundary conditions for matter fields are undeformed. When the matter fields are turned off and the deformation parameter is negative, the mixed boundary conditions for the metric at infinity can be reinterpreted on-shell as Dirichlet boundary conditions at finite bulk radius, in agreement with a previous proposal by McGough, Mezei and Verlinde. The holographic stress tensor of the deformed CFT is fixed by the variational principle, and in pure gravity it coincides with the Brown-York stress tensor on the radial bulk slice with a particular cosmological constant counterterm contribution. In presence of matter fields, the connection between the mixed boundary conditions and the radial "bulk cutoff" is lost. Only the former correctly reproduce the energy of the bulk configuration, as expected from the fact that a universal formula for the deformed energy can only depend on the universal asymptotics of the bulk solution, rather than the details of its interior. The asymptotic symmetry group associated with the mixed boundary conditions consists of two commuting copies of a state-dependent Virasoro algebra, with the same central extension as in the original CFT.

Bidisha Chakrabarty (ICTS-TIFR Bangalore): Nonlinear Langevin dynamics via holography

I will discuss the leading non-linear corrections to the Brownian motion of a heavy quark moving through a strongly coupled CFT plasma (bath) using holographic Schwinger-Keldysh path integral. The bulk spacetime in this formalism is a double copy of AdS black brane with a doubled string that probes it. The string stretches from two AdS boundaries and loops around a `horizon cap' connecting the horizons of the black branes. Evaluating the Nambu Goto action on this doubled string configuration results in the (quartic) influence phase of the heavy quark. The influence phase passes non-trivial consistency conditions arising from the underlying unitarity and thermality of the bath. The local effective theory obeys recently developed non-linear fluctuation dissipation relation that relates the non-Gaussianity of thermal noise to the thermal jitter in the damping constant of the Brownian particle.

Nathan Seiberg (IAS): Field Theories with a Vector Global Symmetry

Motivated by recent discussions of fractons, we will explore nonrelativistic field theories with a continuous global symmetry, whose charge is a spatial vector. We will present several such symmetries and demonstrate them in concrete examples.

Ben Withers (Southampton): Turbulent black holes

I will demonstrate that forced black hole horizons exhibit statistically steady turbulent dynamics consistent with Kolmogorov’s theory of 1941. I will focus on the simplest context for studying black hole dynamics: asymptotically anti-de Sitter spacetimes in a large number of dimensions.

Congkao Wen (QMUL): Flat-space amplitudes, holographic correlators, and supersymmetric localization

In this talk, I will discuss the connections between flat-space amplitudes and holographic correlators via AdS/CFT correspondence. The concrete example we will study is 10D type IIB superstring amplitudes in the \alpha’ expansion and correlation functions in N=4 SYM in the large-N expansion. The 10D type IIB superstring amplitudes at low orders of \alpha’ expansion (BPS ones) are known exactly, both perturbative terms and non-perturbative terms. This talk is to derive such exact results from CFT side. The technique tool we will be using is the supersymmetric localization, which allows us to study the correlators (in 1/N expansion) to all orders in Yang-Mills coupling, which we find to precisely match with the results obtained from 10D type IIB string amplitudes.

Martin Wolf (Surrey): L_\infty-Algebras, Field Theory, and Scattering Amplitudes

I will discuss the Batalin-Vilkovisky formalism for Lagrangian field theories and its mathematical foundations with an emphasis on the higher algebraic structures. In particular, I will explain how a field theory gives rise to an L_\infty-algebra and how quasi-isomorphisms between L_\infty-algebras correspond to physically equivalent field theories. I will also explain how recursion relations for scattering amplitudes a la Berends and Giele can be obtained by transitioning to the smallest representative of the quasi-isomorphism class of that L_\infty-algebra.

Tarek Anous (Amsterdam): A geometric interpretation of the irrelevant JT deformation

A certain class of irrelevant deformations of two dimensional quantum field theories have received some interest over the past few years, since they can surprisingly be exactly solved. The lore behind irrelevant deformations implies that they are plagued by short distance ambiguities which proliferate along the RG flow, in seeming contradiction of this fact. In the specific case of the irrelevant $T\bar{T}$ deformation, a simple explanation for this seeming contradiction is that the deformation can be reinterpreted as coupling the QFT to a precise theory of topological gravity. Since we have added no new degrees of freedom, the solubility of the irrelevant deformation becomes manifest. I will give a brief review the $T\bar{T}$ story and will extend it to another set of irrelevant deformations which explicitly break Lorentz invariance, namely the $JT$ deformation. From this we can match results from the literature, from an intuitive starting point.

Perseas Christodoulidis (Groningen): (Slowly varying) Attractors and Bifurcations in multi-field inflation [slides]

Recent years have seen the introduction of various multi-field inflationary scenarios in which the curvature and geodesics of the scalar manifold play a crucial role. I will outline a simple description that unifies these different proposals and discuss their stability criteria. It will be shown that the underlying dynamics is governed by an effective potential, whose critical points and bifurcations determine the late-time behaviour of the system, thus unifying all different models of this type in the literature. I will also comment on the domain walls analogue of these solutions.

Masanori Hanada (Southampton): Black Hole from Colors [slides]

For several simple theories, we demonstrate the existence of the partially deconfined phase: M of N colors deconfine, and M/N gradually grows from zero (confinement) to one (complete deconfinement). We point out that the mechanism admits a simple interpretation in the form of spontaneous breaking of gauge symmetry. In terms of the dual gravity theory, such breaking occurs during the formation of a black hole. We speculate whether the breaking and restoration of gauge symmetry can serve as an alternative definition of the deconfinement transition in theories without center symmetry, such as QCD. We also discuss the role of the color degrees of freedom in the emergence of the bulk geometry in holographic duality.

Edoardo Lauria (Durham): 3d Abelian Gauge theories at the Boundary [slides]

A four-dimensional abelian gauge theory can be coupled to a 3d CFT with a U(1) symmetry living on a boundary. This coupling gives rise to a continuous family of boundary conformal field theories (BCFTs) parametrized by the gauge coupling \tau and by the choice of the CFT in the decoupling limit. Upon performing an Electric-Magnetic duality in the bulk and going to the decoupling limit in the new frame, one finds a different 3d CFT on the boundary, related to the original one by Witten's SL(2, Z) action. In particular the cusps on the real \tau axis correspond to the 3d gauging of the original CFT. We study general properties of this family of BCFTs. We show how to express bulk one and two-point functions, and the hemisphere free-energy, in terms of the two-point functions of the boundary electric and magnetic currents. Finally, upon assuming particle-vortex duality (and its fermionic version), we show how to turn this machinery into a powerful computational tool to study 3d gauge theories.

Antonin Portelli (Edinburgh): Investigation of holographic cosmological models using lattice simulations [slides]

The recent work arXiv:1607.04878 showed that a holographic description of the early Universe allows one to obtain novel cosmological models, which are competitive with the more standard ΛCDM model to describe our Universe at early times. In this new model, the Cosmic Microwave Background (CMB) spectrum is related to the 2-point function of the trace of the energy-momentum tensor in the dual theory. The fit to Planck data suggests that the dual theory is a gauge theory coupled to scalar fields with a quartic interaction, which while perturbative at high momenta, it becomes non-perturbative at low momenta (low-multipole part of the CMB spectrum). Moreover, the dual theory suffers from severe infrared divergences in perturbation theory, which are believed to be only an artefact of perturbation theory. We propose to address both issues by computing the relevant 2-point functions in dual theory using fully non-perturbative lattice simulations.