In-person, Wednesdays at 1:00pm (student session), 1:20pm (main seminar), unless otherwise stated.
Room location is variable, please check each week.
 

Semester 1  

26/09/23 - Ioannis Papadimitriou (Athens)  -  *Note unusual day*  -  1:00 / 1:20, 54/7033 (7C)

04/10/23 - Prateksh Dhivakar (IIT Kanpur) - 54/7035 (7B)

10/10/23 - David Vegh (QMUL)  -  *Note unusual day* - 1:00 / 1:20, 06/1081 (L/R B)

11/10/23 - Sravan Kumar (Portsmouth) - main seminar 1:00-2:00, student session 2:00-2:20 - 06/1083 (L/R C)

18/10/23 - no seminar - STAG Public Lecture

25/10/23 - Dong-Gang Wang (Cambridge) - main seminar 1:00-2:00, student session 2:00-2:20 - 54/7035 (7B)

01/11/23 - Ioannis Matthaiakakis (Southampton), TBC - 06/1083 (L/R C)

08/11/23 - no seminar - Physics dept job talks

15/11/23 - Masanori Hanada (QMUL) - 54/7035 (7B)

22/11/23 - Antony Speranza (U. Illinois) - main seminar 1:00-2:00, student session 2:00-2:20 - 06/1083 (L/R C)

29/11/23 - Karapet Mkrtchyan (Imperial) - 54/8031 (8C)

05/12/23- Junggi Yoon (APCTP, Pohang)  - 06/1081 (L/R B)

06/12/23 - Monica Kang (UPenn, Philadelphia ) - main seminar 1:00-2:00, student session 2:00-2:20 - 54/7035 (7B)

13/12/23 - Pratik Roy (U. Witwatersrand) - 54/7035 (7B)

[Winter break: 11 Dec 2022 - 12 Jan 2023]

[Exam period: 16-27 Jan 2023]

Semester 2 

31/01/24 - Nico Kovensky (Saclay) - 05/2011 (L/T G) *note unusual location*

07/02/24 - Oscar Dias (Southampton) - 54/5027 (5A)

14/02/24 - no string seminar - [SHEP seminar - Jonathan Oppenheim]

21/02/24 - Kajal Singh (U. Liverpool) - 05/2011 (L/T G) *note unusual location*

28/02/24 - Vaios Ziogas (Bristol U.) - 54/5027 (5A)

06/03/24 - Yoav Zigdon (Cambridge) - 54/5027 (5A)

13/03/24 - Seyed Morteza Hosseini (Imperial) - 54/5027 (5A)

20/03/24 - Micha Berkooz (Weizmann Inst.) - 54/5027 (5A)

[Spring break: 24 March 2022 through 21 April 2022]

24/04/24 - Shota Komatsu (CERN)       - 54/7033 (7C)

01/05/24 - Fridrik Freyr Gautason (Southampton)  - 54/7033 (7C)

08/05/24 - Julius Julius (Harish-Chandra Res. Inst.) - 54/7033 (7C)

15/05/24 - Balt van Rees (École polytechnique) - 54/7035 (7B)

26/06/24 - Emil Martinec (Chicago) - 54/7035 (7B)

Titles and Abstracts (reverse chronological order):

Emil Martinec (Chicago):  Fivebrane Stars

We revisit a construction due to Lunin and Mathur of 1/2-BPS solutions of string theory involving fivebranes, which are the magnetic duals of fundamental strings; and interpret the generic solution as describing a “fivebrane star”.  The effective field theory governing these objects provides insights into gravitational collapse and black hole formation.

Balt van Rees (École polytechnique):  QFT in AdS instead of LSZ

The boundary correlation functions for a QFT in a fixed AdS background should reduce to S-matrix elements in the flat-space limit. We consider this procedure in detail for four-point functions. With minimal assumptions we rigorously show that the resulting S-matrix element obeys a dispersion relation, the non-linear unitarity conditions, and the Froissart-Martin bound. QFT in AdS thus provides an alternative route to fundamental QFT results that normally rely on the LSZ axioms.

Julius Julius (Harish-Chandra Res. Inst.): CFT-data of N=4 Super-Yang-Mills

We discuss 4D N=4 Super-Yang-Mills (SYM) theory in the planar

limit. We pose the question: how to find the CFT-data of the theory? The

anomalous dimensions of single-trace "stringy" super conformal primary

operators can be numerically computed using the integrability-based

Quantum Spectral Curve method, and we go through the recent results. We

highlight the strong coupling limit of the theory where the spectrum

possesses an interesting structure and organises into the KK-towers. We

also discuss lifting the degeneracy of the structure constants of the

"stringy" operators at strong coupling and describe the observed

patterns. Combining our spectral results with recent advances in

computation of the AdS Virasoro-Shapiro amplitude, we are able to

extract predictions for many unprotected OPE coefficients of N=4 SYM at

strong coupling. We comment on certain curiosities and patterns observed in the obtained predictions.

Preseminar details: I will give an intro to integrability in N=4 SYM followed by some flavour of the Quantum Spectral Curve method.

Fridrik Freyr Gautason (Southampton): S-folds and their conformal manifolds

I discuss a 3D N=4 SCFT that is constructed by gauging a global symmetry of the Gaiotto-Witten T[U(N)] theory. Supersymmetric localization allows for the exact evaluation of its sphere partition function which reveals a remarkably simple answer that can be interpreted as a giant graviton expansion. I give some credence to this interpretation in a direct holographic study of their S-fold dual. The holographic dual shows the existence of a dim_C=1 (non-compact) conformal manifold that preserves N=2 supersymmetry and a dim_C=2 conformal manifold where all susy is broken. By further holographic study we can compute at large rank, the (low lying) spectrum of primary operators, the partition functions on compact 3-manifolds and study their line operators.

Shota Komatsu (CERN) : Non-Invertible Symmetries, Anomalies and Scattering Amplitudes

We show that crossing symmetry of S-matrices is modified in certain theories with non-invertible symmetries or anomalies. Focusing on integrable flows to gapped phases in two dimensions, we find that S-matrices derived previously from the bootstrap approach are incompatible with non-invertible symmetries along the flow. We present consistent alternatives, which however violate standard crossing symmetry and obey modified rules dictated by fusion categories. We extend these rules to theories with discrete anomalies. 

Micha Berkooz (Weizmann Inst.): The double scaled SYK model: quantum chaos and non-commutative spacetime.

The double scaled Sachdev-Ye_Kitaev is a p-local quantum mechanical system of N (being large) Majorana fermions, with a low energy non-Fermi liquid phase. It is dual to gravity on AdS2, which is described by JT gravity at low energies. In the double scaled SYK, in which we scale the parameter p as N^{1/2},  we can solve the theory completely at all energy scales just using combinatorics. The procedure automatically generates the boundary particle dynamics, ie., key aspects of the bulk theory. We will discuss the underlying q-deformed SL(2) structure, what it means for the existence of a minimal length in spacetime, how it turns spacetime into a non-commutative space, and how to generalize the technique to study new phases of the theory such integrable to chaotic transitions. 

Seyed Morteza Hosseini (Imperial): Gravitational Blocks and Matrix Models

Investigating the fundamental origins of Bekenstein-Hawking entropy remains a profound challenge within the realm of theoretical physics. This exploration is particularly pertinent to anti de Sitter (AdS) black holes, where it is posited that the entropy in question may be explained via the states of the holographic dual quantum field theory, in accordance with the AdS/CFT correspondence. In my talk, I will introduce the idea of gluing gravitational blocks for supersymmetric AdS black holes in String/M-theory, encompassing any spin and a wide array of electric and magnetic charges. This approach provides insights into the behavior of the partition function for the corresponding holographic dual field theory at large N.

Yoav Zigdon (Cambridge):  Rotating Horowitz-Polchinski Solutions

We utilize analytical and numerical methods to find a smooth, stationary rotating solution in the heterotic string theory at high temperatures. The solution describes a spinning winding-momentum condensate living in three non-compact dimensions, and its backreaction on the thermal cycle. At low-temperatures, we expect a transition between our solution to an analytical continuation of an axionic Kerr black hole. To appear in the near future, with Jorge Santos.


Vaios Ziogas (Bristol U.):  U(1) quasi-hydrodynamics: Schwinger-Keldysh effective field theory and holography

In nature symmetries are softly broken in a wide range of systems, from axial charge relaxation in QCD to pinned charge density wave states in condensed matter. As a toy model for more realistic situations, in this talk I will study the low energy physics of a system with an approximate U(1) global symmetry, using effective field theory and holographic methods. I will begin by discussing the construction of Schwinger-Keldysh effective hydrodynamic actions, and I will apply these techniques in the case of hydrodynamics with U(1) charge relaxation. I will then explain how one can derive this SK effective action by performing a holographic analysis of a bulk Proca model.

Kajal Singh (U. Liverpool):  Finding G_2 Higgs Branch of 4D rank 1 SCFTs

In this talk, I will discuss the correspondence between 4D N=2 unitary SCFTs and 2D non-unitary CFTs. The rank one case has been shown to lead to the non-unitary CFTs with Deligne-Cvitanovic (DC) exceptional sequence of Lie groups. Utilizing our proposed method of unitarization, I will demonstrate that the unflavored characters of the DC CFTs map to the characters of the Mathur-Mukhi-Sen Sequence. Then, focusing on the example of G_2/E_6 correspondence, we demonstrate that our unitarization map holds for some discrete flavor fugacities as well. Lastly, using Galois conjugation along with our result, we propose that the G_2 Higgs branch is a sub-branch of the E_6 Higgs branch.

The talk will be based on https://arxiv.org/abs/2312.00275.

Oscar Dias (Southampton): New SYM phases at finite chemical potential 

We do a systematic search of supergravity solutions that, via the AdS5/CFT4 correspondence, are dual to thermal states in N=4 SYM at finite chemical potential. These solutions are required to ultimately reproduce the microscopic entropy of AdS black holes. Using a mix of analytical and numerical methods, we construct and study static and rotating charged hairy solitonic and black hole solutions with global AdS5 asymptotics. They are constructed in a consistent truncation of five dimensional SO(6) gauged supergravity and can thus be uplifted to asymptotically AdS5 x S5 solutions of type IIB supergravity (it is also a truncation of N=8 gauged supergravity). Hairy black holes exist above a critical electric charge and merge with the known Cvetic-Lu-Pope (CLP) black holes along a curve determined by the onset of superradiance in the latter family. The hairy black holes then extend all the way up to the BPS limit (in a phase diagram) and they dominante the microcanonical ensemble when they coexist with the CLP black holes. In the BPS limit, our finite temperature black holes eventually attempt to approach new supersymmetric hairy black holes that reduce to the supersymmetric Lucietti-Kunduri-Reall black hole family when the hair condensate vanishes. Our findings permit a good understanding of the full phase space of SYM thermal states with three arbitrary chemical potentials and finite charged scalar fields. 

Nico Kovensky (Saclay) :  Ascending the attractor flow in the D1-D5 system 

We study maximally supersymmetric irrelevant deformations of the D1-D5 CFT that correspond to following the attractor flow in reverse in the dual half-BPS black string solutions of type IIB supergravity on K3. When a single, quadratic condition is imposed on the parameters of the 22 such irrelevant deformations, the asymptotics of the solution degenerate to a linear dilaton-like spacetime. We identify each such degeneration limit with a known decoupling limit of string theory, which yields little string theory or deformations thereof (the so-called open brane LST, or ODp theories), compactified to two dimensions.  This suggests that a 21-parameter family of the above deformations leads to UV-complete theories, which are string theories decoupled from gravity that are continuously connected to each other.  All these theories have been argued to display Hagedorn behaviour; we show that including the F1/D1 sources leads to an additional Cardy term. 

Pratik Roy (U. Witwatersrand):  Quantum thermodynamics from holographic quenches 

Quantum entanglement has been the focus of a lot of research over the past couple of decades. In particular, a new bound on the second derivative of relative entropy in QFTs has emerged from the study of holographic theories. This bound, called the Quantum Null Energy Condition (QNEC), has been proven rigorously in a very general setting. In the context of a two-dimensional quantum conformal field theory that undergoes a sudden injection of energy (i.e., a global quench), we will show that QNEC can be used to place bounds on, e.g., the amount of increase of entropy in terms of the increase in temperature. The bound obtained is stronger than the Clausius inequality of classical thermodynamics, which is necessary but not sufficient to ensure that QNEC is not violated. 

Monica Kang (Caltech): Emergent N=4 supersymmetry from N=1 

I will construct 4d N=1,2 SCFTs with identical central charges a=c (without a large N limit) via the diagonal gauging of collections of non-Lagrangian Argyres–Douglas and conformal matter theories. Utilizing a particular family of theories from this construction, I will present a four-dimensional N=1 supersymmetric field theory that is dual to the N=4 super Yang—Mills theory with gauge group SU(2n+1) for each n. The dual theory is constructed through the diagonal gauging of the SU(2n+1) flavor symmetry of three copies of a particular Argyres–Douglas theory. This theory flows in the infrared to a strongly-coupled N=1 SCFT that lies on the same conformal manifold as N=4 SYM. 

Junggi Yoon (APCTP, Pohang): Gravitational edge mode for 2D gravity.

In this seminar, I will discuss the gravitational edge mode of the two-dimensional gravity. First, I will review the gravitational edge mode from the bosonic JT gravity. I will discuss the issues related to dilaton and PSL(2,R) gauging in the derivation of the Schwarzian action. I will revisit the derivation of Schwarzian action for the edge mode by using the inversion formula of Schwarzian derivative. I will explain how to obtain N=1 super-Schwarzian action. I will also discuss the relation between osp(2,1) gauging and isometry.

Karapet Mkrtchyan (Imperial): Democratic Lagrangians and where they come from 

I will summarize the work of the last few years, culminating at covariant Lagrangian formulation of arbitrary abelian interactions of chiral p-forms and democratic (treating electric and magnetic degrees of freedom on equal footing) formulation of abelian interactions of arbitrary p-forms in arbitrary dimensions. This includes manifestly covariant and SO(2) duality invariant Lagrangians of all duality-symmetric electrodynamics in 3+1 dimensions and democratic type II Supergravities in 10d. I will also demonstrate a derivation for these Lagrangians from simple topological theories in one higher dimension. 

Antony Speranza (U. Illinois): Generalized entropy for general subregions in quantum gravity 

I will describe a construction of algebras of observables associated with local subregions in quantum gravity in the small G_N limit.  This algebra consists of operators dressed to a semiclassical observer degree of freedom which serves as an anchor defining the subregion.  I will argue that properly implementing the gravitational constraints on this algebra results in a type II von Neumann algebra, which possesses a well-defined notion of entropy.  Up to a state-independent constant, this entropy agrees with the UV-finite generalized entropy of the subregion, consisting of a Bekenstein-Hawking area term and a bulk entropy term.  This gives an algebraic explanation for the finiteness of the generalized entropy, and provides a number of tools for investigating aspects of semiclassical gravitational entropy, including the generalized second law, the quantum focusing conjecture, and the quantum extremal surface prescription in holography. 

Masanori Hanada (QMUL): Color Confinement and Random Matrices

We give a microscopic mechanism of confinement in gauge theories including pure Yang-Mills and QCD, elaborating on "Confinement = BEC" mechanism proposed by Shimada, Wintergerst and MH. As a byproduct, we show a nontrivial behavior of Polyakov lines in the low-temperature regime of confining gauge theories: Polyakov lines are slowly varying Haar random modulo corrections exponentially small with respect to the inverse temperature. With exact Haar randomness, computation of two-point correlator of Polyakov loops reduces to the problem of random walk on group manifold. We show that linear confinement potential with approximate Casimir scaling of string tension follows naturally from such a random walk except for short distance. With exponentially small corrections to Haar randomness, string breaking and loss of Casimir scaling at long distance follow. Therefore, we obtain a Casimir scaling which is only approximate and valid only at intermediate distance --- This is precisely what we need to explain the results of lattice simulations! 

If time permits, I explain some other results on finite-N theories, too. 

This talk is based on my recent paper with Hiromasa Watanabe (arXiv:2310.07533, "On thermal transition in QCD") and a paper in preparation with Georg Bergner and Vaibhav Gautam. 

Ioannis Matthaiakakis (Southampton): Quantum Complexity as Hydrodynamics 

Quantum circuit complexity is a measure of the implementation difficulty of unitary operators acting on a Hilbert space. While originally defined for finite dimensional Hilbert spaces, complexity has recently been argued to play an important role in understanding  (holographic) QFTs. Despite this, we lack a clear and well-defined connection between the complexity of systems with finite and infinite degrees of freedom. 

In this talk, I will show that there exists such a connection for the group of SU(N) unitaries as N goes to infinity. In particular I will show that generating SU(N) unitaries via a set of non-commutative plane waves and penalizing harsher the operators with larger wave-momentum, leads to a well-defined and tractable large-N limit. In this limit, we can use the Euler-Arnold approach to show that 2d inviscid incompressible hydrodynamics on a 2-torus emerges as an effective theory of complexity.  Examining complexity from the point of view of this effective theory, indicates that for large N our complexity measure captures two essential properties of holographic complexity measures: ergodicity and conjugate points.

Dong-Gang Wang (Cambridge): On the IR divergences in de Sitter: from trees to loops and back 

Interacting light scalars in de Sitter space (dS) normally lead to infrared (IR) divergences. I shall revisit this topic with recent developments of the cosmological bootstrap: 

At the tree level, we first see that massive exchange diagrams are IR-finite, and conformal Ward identities lead to the boundary differential equations, which in the end allows us to bootstrap a full set of cosmological collider non-Gaussianity. While for massless exchanges, IR divergences are normally expected, and then the boundary differential equations become the ones from anomalous conformal Ward identities. This leads to a full classification for non-Gaussianities from multi-field inflation. 

At the loop level, we apply the wavefunction method, and identify that the leading contributions to IR-divergent correlators always come from classical loops with tree-level wavefunction coefficients. This significantly simplifies the problem and indicates the importance of the saddle-point approximation when we go beyond perturbation theory. With the new insight, we present a non-perturbative derivation of the stochastic formalism. Using the semi-classical wavefunction, we find that the Fokker-Planck equation follows as a consequence of the Schroedinger equation and the Polchinski's equation for the exact renormalization group flow.

Sravan Kumar (Portsmouth): Non-localities in quantum gravity and their implications for early Universe cosmology and black hole physics

Unitarity and renormalizability are the two most important tools to guide us toward UV completion of gravity. In this view, I will discuss two types of spacetime non-localities that play a crucial role in our understanding of gravity at the short distance scales and at the horizon scales. The first kind of non-locality is associated with non-point-like interactions of fundamental degrees of freedom towards the Planck scales. I will present in detail the implications of this non-locality to the early Universe cosmology and to the primordial gravitational waves. The second non-locality is associated with the nature of quantum fields in curved spacetime. In this regard, I will present a new formulation of a unitary quantum field theory in curved space-time with its observational signatures in the context of inflationary cosmology and its implications for the understanding of black hole evaporation.

David Vegh (QMUL):  The 't Hooft equation as a quantum spectral curve

In this talk, I examine the massless 't Hooft equation. This integral equation governs meson bound state wavefunctions in 2D SU(N) gauge theory in the large-N limit, and it can also be obtained by quantizing a folded string in flat space. The folded string is a limiting case of a more general setup: a four-segmented string moving in three-dimensional anti-de Sitter (AdS) space. I compute its classical spectral curve using celestial variables and planar bipartite graphs, also known as on-shell diagrams or brane tilings. In this more general setup, the 't Hooft equation acquires an extra term, which has previously been proposed as an effective confining potential in QCD. After an integral transform, the equation can be inverted in terms of a finite difference equation. I show that this difference equation has a natural interpretation as the quantized (non-analytic) spectral curve of the string. The spectrum interpolates between equally spaced energy levels in the tensionless limit and 't Hooft's nearly linear Regge trajectory at infinite AdS radius.

Prateksh Dhivakar (IIT Kanpur):  AdS Witten diagrams to Carrollian correlators

Carrollian Conformal Field Theories (CFTs) have been proposed as co-dimension one holographic duals to asymptotically flat spacetimes as opposed to Celestial CFTs which are co-dimension two. In this talk, we start with a brief introduction to this approach to flat space holography. Afterwards, drawing inspiration from Celestial holography, we show by a suitable generalisation of the flat space limit of AdS that keeps track of the previously disregarded null direction, one can reproduce Carrollian CFT correlation functions from AdS Witten diagrams. In particular, considering Witten diagrams in AdS$_4$, we reproduce two and three-point correlation functions for three dimensional Carrollian CFTs in the so-called delta-function branch. We also obtain a generalised anti-podal matching condition that now depends on the retarded time direction. This establishes a direct link between AdS holography and flat holography. Based on arXiv:2303.07388 [hep-th].

Ioannis Papadimitriou (Athens):  Algebraically special solutions and accelerating black hole thermodynamics

Algebraically special solutions constitute a broad class of generally time dependent but analytically known solutions of Einstein's equations. The physics they describe, however, remains relatively less understood, mainly due to the boundary conditions they satisfy at asymptotic infinity. In this talk I will review algebraically special solutions with a negative cosmological constant in four dimensions and will present accelerating black holes as a particular example. I will then discuss aspects of the holographic dictionary for such solutions and will apply it to the thermodynamics of asymptotically locally AdS black holes.