Joint Belgian hep-th seminars: academic year 2022-2023
31 May 2023, Mons
Michele Schiavina (U. Pavia) | Hamiltonian gauge theory with corners. Soft symmetries, memory and superselection from Hamiltonian reduction by stages
A general paradigm in classical mechanics is that a mechanical system with integrable (i.e. Hamiltonian) symmetries can be reduced by fixing a value of the associated conserved quantity. When the symmetry group admits a normal subgroup, reduction can be done in steps, and one gets “intermediate phase spaces” that can be useful for various applications. This procedure is called Hamiltonian reduction by stages, and it extends to gauge field theory, where it becomes subtler due to the symmetries being local. If one takes the symplectic formulation of gauge theory seriously, by viewing the construction of the reduced phase space of the theory as an example of Hamiltonian reduction, a number of interesting observations ensue when the field theory is defined on manifolds with corners, enabling Hamiltonian reduction by stages due to the existence of the (normal) subgroup of gauge transformations that are “trivial at the corner”. By implementing Hamiltonian reduction by stages within the local gauge theory scenario, I will construct the reduced phase space of a large class of gauge theories, and show how a number of topics in high-energy physics, such as the existence of soft/asymptotic symmetries (and the conservation of associated charges), as well as various “memory effects” can be seen as a straightforward, albeit highly nontrivial, application of Hamiltonian reduction by stages. This talk is based on two joint works with A. Riello: 2207.00568 and 2303.03531.
Sašo Grozdanov (University of Edinburgh and University of Ljubljana) | Spectra, reconstructions and pole-skipping
The poles of two-point functions in momentum space, which can be computed and analysed using holographic methods, reveal various details of the physical properties of spectra in QFTs. In thermal QFTs, the lowest-energy (IR) gapless mode is usually described by the theory of hydrodynamics. Assuming a known dispersion relation of only a single hydrodynamic mode, I will discuss when and how the reconstruction of the complete spectrum of physical excitations is possible in the corresponding correlator. In particular, I will demonstrate our recently developed constructive algorithm based on the theorems of Darboux and Puiseux that allows for a reconstruction of all modes connected by `level-crossings’ in the associated spectral curve, from IR to UV. In the second part of my talk, I will introduce the phenomenon of pole-skipping (formally, a “0/0”) in such correlators and discuss how its knowledge can itself be sufficient to reconstruct the entire spectrum.
24 May 2023, Brussels
Gary Shiu (University of Wisconsin-Madison & BEL Professor) | Quantum Gravity and the Swampland
String theory seems to offer an enormous number of possibilities for low energy physics. The huge set of solutions is often known as the String Theory Landscape. In recent years, however, it has become clear that not all quantum field theories can be consistently coupled to gravity. Theories that cannot be ultraviolet completed in quantum gravity are said to be in the Swampland. In the first two lectures (May 19), I’ll discuss the motivation behind the Swampland program and present some of the better established Swampland criteria such as the absence of exact global symmetries, the various formulations of the Weak Gravity Conjecture(s) and their physical consequences. The axionic version of the Weak Gravity Conjecture also naturally connects Swampland questions with Euclidean wormholes. In Lectures 3 and 4 (May 24), I will focus on challenges in realizing accelerating universes from string theory. I’ll begin with the Dine-Seiberg problem which emphasizes the tension between moduli stabilization in string theory and control of corrections to the effective field theory. Then, I’ll turn to the de Sitter conjecture which highlights further challenges in finding metastable vacua with positive vacuum energy. I’ll end by presenting some recent findings which elucidate the late-time behaviors of cosmological solutions and explain what these behaviors mean for realizing accelerating universes. Throughout the lectures, I hope to convey how the Swampland program meets other areas of high energy theory, such as holography, scattering amplitudes, bootstrap, black hole physics, and dynamical systems.
17 May 2023, Leuven
Alfredo González Lezcano (APCTP, Pohang) | Supersymmetric localization on AdS2
Application of the supersymmetric localization method to theories on AdS space has received recent interest yet remains a challenging problem. The main obstacle is that supersymmetric transformations do not map into each other the bosonic and fermionic standard delta-function normalizable basis in terms of which the quantum fluctuations are typically expressed. We overcome this problem by constructing a supersymmetric Hilbert space for scalar and spinor fields on Euclidean global AdS2 by considering the complexified spectrum of the Dirac operator. The new basis remains delta-function normalizable with an appropriate inner product and is compatible with the asymptotic boundary condition demanded by the variational principle. Once we have a supersymmetric Hilbert space, we can safely evaluate the path integral using localization. We then focus on (global) Euclidean AdS2, on which we consider an Abelian N=(2,2) theory and implement localization computation to obtain the exact partition function. To evaluate the 1-loop determinant about the localization saddle, we use index theory and fixed-point formula, where we pay attention to the effect of zero modes and their superpartners. The resulting partition function reveals an overall dependence on the size of the background manifold as a sum of two types of contributions: a local one from local conformal anomaly through the index computation and a global one coming from zero modes. This overall size dependence is confirmed by the perturbative 1-loop evaluation using heat kernel method.
Victor Gorbenko (EPFL) | Quantum Fields in de Sitter in Non-perturbative regimes
I will discuss several interconnected topics in inflationary cosmology: dynamics of light fields in dS and in inflation and taming of IR divergences; conformal-partial-waves decomposition of cosmological correlators, constraints that unitarity imposes on it and a formulation of a bootstrap problem.
10 May 2023, Brussels
Ronak Soni (Cambridge University) | Towards Tensor Networks with Non-Commuting Areas
There has been much interest recently in discrete toy models of hoolography called tensor networks. This is based primarily on the fact that entanglement between boundary regions is given by a minimal cut of the network, in a way reminiscent of the Ryu-Takayanagi formula in the case of a static bulk geometry. These models have two related shortcomings: they can’t reproduce the full maximin condition relevant in the general case, and they can’t reproduce the fact that the areas of two intersecting RT surfaces are non-commuting operators in the semi-classical theory. This can be traced back to the fact that the tensor networks don’t satisfy any discretised version of the Hamiltonian constraint of gravity. To remedy this situation, we go back to pure GR in three dimensions, which can be rewritten as a topological Chern-Simons theory. A lattice version of Chern-Simons theory, known as the quantum double model or the Levin-Wen model, naturally satisfies a Hamiltonian constraint and also has an analog of non-commuting areas. We propose that these models can be used as a starting point for building holographic tensor networks.
Marine De Clerck (Cambridge University) | Integrability and complexity in quantum spin chains
There is a widespread perception that dynamical evolution of integrable systems should be simpler in a quantifiable sense than the evolution of generic systems, though demonstrating this relation between integrability and reduced complexity in practice has remained elusive. The notion of complexity of interest to us will be Nielsen’s complexity applied to the time-evolution operator of quantum systems. I will start by reviewing Nielsen’s complexity, discuss the difficulties associated with this definition and introduce a simplified approach that we first considered in arXiv:2202.13924, which appears to retain non-trivial information about the integrable properties of the dynamical systems. Subsequently, I will present recent results from arXiv:2305.00037, where we studied the correlation between integrability and complexity reduction in detail using quantum spin chains which possess diverse integrable structures.
3 May 2023, Leuven
Matilda Delgado (Madrid, IFT) | Probing far away regions of moduli space
I will discuss the difficulties one encounters in attempting to probe (near-) infinite distances in moduli space using local solutions of the EFT. I will first argue that one generically runs into spacetime singularities due to the back-reaction on spacetime of such large field excursions. I will then argue that one can solve this issue by considering large black holes. I will focus on 4d N = 2 BPS black holes and use their attractor mechanism to show how one can extract topological data about the underlying compactification by measuring black hole thermodynamic observables. Furthermore, I will illustrate how the standard distance in moduli space can be related asymptotically to the black hole mass and entropy. I will also compute a measure of the resolution with which BPS black holes of a given mass can distinguish far away points in the moduli space. Based on: 2212.08676, 2203.11240.
Iñaki García Etxebarria (Durham U.) | Branes and symmetries for N=3 SCFTs
I will explain how to derive various results about the generalised symmetry structure of N=3 and N=4 S-fold SCFTs using holography. The new results in the N=3 case include the spectrum of 1-form symmetries, their ‘t Hooft anomalies, and the existence of non-invertible symmetries for appropriate choices of global form.
19 April 2023, Brussels
Martín Sasieta (Brandeis University) | Microscopic origin of the entropy of black holes in general relativity
In many situations, the local description of the interior of the black hole is in tension with its universal entropy, given by the Bekenstein-Hawking formula. In this talk, I will start by constructing an infinite family of semiclassical microstates with distinct geometric interiors in the gravitational effective field theory, which naively overcount the entropy of the black hole. I will then show that the gravitational path integral is able to detect non-vanishing overlaps between these states via exponentially suppressed, yet universal, wormhole contributions. I will finally show two ways to use this information to conclude that the Hilbert space spanned by these states is e^S dimensional, where S is the Bekenstein-Hawking entropy of the black hole. I will end with some open questions and future directions.
Christoph Uhlemann (University of Oxford) | Double holography and Page curves in Type IIB
Randall-Sundrum braneworld models provide a fruitful setting to study gravitational systems coupled to a QFT bath through the concept of double holography, and they are key ingredients in recent studies of the black hole information paradox. However, these bottom-up models are qualitative and their holographic interpretation plagued by causality problems. In this talk we discuss top-down models realizing 4d black holes coupled to a QFT bath in Type IIB string theory and obtain Page curves consistent with unitarity. We make the concept of double holography precise and resolve the causality problems found in the bottom-up models, leading to a refinement of their interpretation.
29 March 2023, Leuven
Piotr Tourkine (Annecy, LAPTh) | Scattering amplitudes from dispersive iterations of unitarity
In 1968, D. Atkinson proved in a series of papers the existence of functions satisfying all known constraints of the S-matrix bootstrap for the 2-to-2 S-matrix of scalar, gapped theories, following an approach suggested by Mandelstam. To date, this is the only result of this sort, while a contrario no current technology allows to generate, even numerically, fully UV-complete S-matrices in d>2. Beyond the mathematical results themselves, the proof, based on establishing the existence of a fixed point of a certain map, also suggests a procedure to be implemented numerically and which would produce fully consistent S-matrix functions via iterating dispersion relations, and using as an input a quantity related to the inelasticity of a given scattering process. In this talk, I will present the results of a recent paper in collaboration with A. Zhiboedov, about the first implementation this scheme. I will first review basic concepts of the S-matrix program, and state our working assumptions. I will then present our numerical non-perturbative S-matrices, and discuss some of their properties. They correspond to scalar, massive phi^4-like S-matrices in 3 and 4 dimensions, and have interesting and non-trivial high energy and near-threshold behaviour. They also allow to make contact with the running of the coupling constant. I will also compare to other approaches to the S-matrix bootstrap in the literature.
Stefano Cremonesi (Durham U.) | Toric BPS quivers, non-isolated singularities, and deformations
Toric quiver gauge theories describe the worldvolume dynamics of Dp-branes probing (3-complex dimensional) toric Calabi-Yau cones. About twenty years ago, the p=3 case was studied in depth due to its relevance to supersymmetric AdS_5/CFT_4 dualities. More recently, the p=0 case has received interest as it encodes BPS particle excitations of 5-dimensional superconformal field theories on a circle. In this talk I will describe how non-isolated singularities in the geometry (and some of their deformations) impact the 5-dimensional SCFTs and their BPS quivers. If time allows, I will also make some comments on Higgs branch chiral rings of 5d SCFTs.
22 March 2023, Leuven
Pablo Bueno (Universitat de Barcelona) | Quantum field theory from entanglement
Growing evidence suggests that vacuum entanglement entropy (EE) may provide a universal description of QFT (in general dimensions), alternative to correlation functions. In this talk I will review the set of known axioms satisfied by EE in QFT and some of its successes in the direction of uniquely characterising the different models. Then, I will argue that such set of axioms is at present incomplete by explicitly showing that certain models which satisfy all the axioms do not correspond to any actual QFT.
Fridrich Valach (Imperial College London) | Algebroids for membranes, strings, and particles
I will discuss G-algebroids, structures related to various supergravities appearing in string and M-theory. I will describe some basic properties of these algebroids, present the main classification results, and show how they are useful for understanding Poisson-Lie U-duality and consistent truncations of 11D or 10D type IIA/B supergravities. This is a joint work with M. Bugden, O. Hulik, and D. Waldram.
15 March 2023, Brussels
Boris Pioline (Paris, LPTHE) | Counting Calabi-Yau black holes with (mock) modular forms
Unlike in cases with maximal or half-maximal supersymmetry, the spectrum of supersymmetric black holes in type II string theory compactified on a Calabi-Yau threefold with generic SU(3) holonomy remains partially understood. Mathematically, the BPS indices counting these states coincide by the generalized Donaldson-Thomas invariants associated to the derived category of coherent sheaves, but they are rarely known explicitly. String dualities indicate that suitable generating series of rank 0 Donaldson-Thomas invariants counting D4-D2-D0 bound states should transform as vector-valued mock modular forms, in a very precise sense. I will spell out and test these predictions in the case of one-modulus compact Calabi-Yau manifolds (such as the quintic threefold), where the BPS indices can (at least in principle) be computed from Gopakumar-Vafa invariants, using recent mathematical results by S. Feyzbakhsh and R. Thomas.
Diego Rodriguez-Gomez (Universidad de Oviedo) | Field theory defects through double scaling limits
Defect operators in field theory are very interesting for a number of reasons. Drawing inspiration from techniques which have been very recently applied to uncover interesting properties of sectors of operators with large charge under a global symmetry, we will study defects in the Wilson-Fisher fixed point near d=4,6 dimensions. Combining with localization, we will also use a double-scaling limit for certain Wilson loops in N=2 supersymmetric theories in 4d which allows to make exact statements at finite N.
8 March 2023, Ghent
Lea Bottini (University of Oxford) | Non-invertible symmetries and categorical structures
Non-invertible symmetries in various dimensions have received a lot of attention recently. In this talk I will review some of their constructions and apply them to concrete gauge theory examples. I will also explain how in general the symmetries of a quantum field theory are described by a higher category and discuss some interesting phenomena that arise studying gaugings of symmetries in this context.
Jacopo Papalini (INFN, Parma) | Nonperturbative aspects of JT gravity and TT deformation
In this talk, I will investigate nonperturbative effects and phenomena occurring in low-dimensional gravity and gauge theories, both in presence and absence of integrable deformations. Specifically, I will concentrate on JT gravity, one of the rare examples of exactly solvable quantum gravity models, describing how the inclusion of nonperturbative completions is essential to render physical quantities well-definite, expecially when trying to extend the construction to higher-genus topologies in the gravitational path integral. For this purpose, I will then focus on the finite cutoff formulation of JT gravity, which from the holographic perspective is realized by a TT deformation of the dual Schwarzian theory on the asymptotic boundary. In the second part of the talk, I will introduce general properties of the TT deformation and move on by solving the emerging puzzling features about the deformed physical spectrum in the concrete case of TT-deformed Yang-Mills theory. I will derive the exact partition function for U(N) gauge group on the sphere topology, by studying the theory at the level of individual flux sectors. Finally, I will show how, in the large-N expansion, the theory exhibits an intriguing phase diagram where the transitions between different phases are driven by instantons both in the ‘t Hooft coupling and in the TT deformation parameter.
1 March 2023, Brussels
Rishi Mouland (Cambridge U., DAMTP) | Black Holes in the Dual of Quantum Mechanics
I will motivate and study a holographic duality between on one hand a theory of superconformal quantum mechanics, and on the other, M-theory on a particular background. I will exhibit a broad family of black hole solutions in the bulk theory, which fall into a wider class of “ultra-spinning” black holes in various backgrounds. Through an asymptotic study of the superconformal index in the supergravity regime, I will provide a precise microstate counting for those ultra-spinning black holes that are supersymmetric. Finally, I will briefly discuss the relation of conformal quantum mechanics and ultra-spinning black holes to notions of holomorphic factorisation and gravitation blocks, for holographic CFTs in four and six dimensions.
Horatiu Nastase (Sao Paulo, IFT) | Learning from Penrose limits of less understood gravity dual pairs
The Penrose limit of AdS/CFT, giving the pp wave correspondence, simplifies the map between gravity dual pairs; in particular, one can usually quantize string eigenmodes. In the original application to AdS_5xS^5, one obtained a test of AdS/CFT in a nontrivial regime. But it is also useful to apply the limit in order to better understand gravity dual pairs where the map, or the field theory, are less understood. I show results for the cases of the Guarino, Varela, Jafferis (GJV) duality, the T-dual of AdS_5xS^5, the Maldacena-Nastase (MNa) case and its T-dual.
22 February 2023, Brussels
Luca Iliesiu (Stanford University) | Indices from the gravitational path integral – new forms of attraction
Recent results have revealed that black holes close to extremality experience large quantum corrections that drastically affect the black hole entropy at low temperatures. Because of this, all extremal solutions discussed in the past need to be reanalyzed. In this talk, I will explain how to understand the contributions to the gravitational partition function of two such solutions: the attractor solution and multi-center black hole solutions. Our new understanding leads to a new perspective on how the attractor mechanism can be understood at finite temperatures in Euclidean signature.
Oliver Janssen (ICTP, Trieste) | Positivity in EFTs with spontaneously broken boosts
We will discuss positivity bounds on EFT coefficients in theories where Lorentz boosts are spontaneously broken. The well-known S-matrix argument from the Lorentz-invariant scenario does not straightforwardly generalize to this case. Instead the analytic properties of the retarded Green’s function of conserved currents will be used, and the theory will be assumed to become conformal in the UV. The method is general and applicable to both cosmology and condensed matter systems. As a concrete example we will consider the EFT of conformal superfluids, which describes the universal low-energy dynamics of CFTs at large U(1) charge, and we will derive inequalities on the coefficients of the operators, in three spacetime dimensions, at NLO and NNLO.
15 February 2023, Brussels
Ivano Basile (ASC, Munich) | SUSY-less AdS: braneworlds and scale separation
Model building based on string compactifications faces difficult challenges at each step: moduli can be tricky to stabilize, scales to separate, dark energies to uplift and supersymmetry to break in a controlled fashion. String theories where the latter is broken at the string scale can offer different corners of the landscape to explore, with different pros and cons. I will describe how the instabilities of these models leave room for potentially realistic braneworld cosmologies, focusing on the simplest known examples.
Shani Meynet (Uppsala University) | McKay quivers, a physical perspective
In this talk I will discuss McKay quivers and their application in theoretical physics. After a review of the main mathematical results, I will discuss their physical interpretation and, in particular, I will focus on two main aspects: the study of BPS states and higher form symmetries, in the context of geometric engineering, and the phenomenon of decomposition. Both of these aspects rely on the fact that the target space of the string sigma model is an orbifold of C^3 modded by a finite subgroup of SU(3), and thus can be studied in term of McKay quivers.
8 February 2023, Leuven
Axel Kleinschmidt (AEI Potsdam) | BEL lecture: String scattering old and new III
This lecture will discuss the systematics of low-energy expansions, first focussing on one-loop amplitudes and so-called modular graph functions and then on non-perturbative aspects and U-duality.
Emanuel Malek (Humboldt-Universität zu Berlin) | Kaluza-Klein Spectrometry for String Theory Compactifications
I will present a powerful new method that for the first time allows us to compute the Kaluza-Klein spectrum of a large class of string theory compactifications. This includes geometries with little to no remaining (super-)symmetries, completely inaccessible by previous methods. I will show how these insights can be used to compute the anomalous dimensions of protected and unprotected operators in strongly-coupled CFTs, as well as to study global properties of their conformal manifolds. I will also show how the method can be used to determine the perturbative stability of non-supersymmetric AdS vacua. This reveals that tachyons can arise from higher Kaluza-Klein modes that are not visible in lower dimensions. Intriguingly, there are also classes of fully perturbatively stable non-supersymmetric AdS vacua in 10-d supergravity.
Axel Kleinschmidt (AEI Potsdam) | BEL lecture: String scattering old and new IV
1 February 2023, Mons
Axel Kleinschmidt (AEI Potsdam) | BEL lectures: String scattering old and new
Lecture I: I will review some of the basic foundations of string scattering amplitudes, including CFT techniques, Riemann surfaces and leading to the Veneziano and Virasoro-Shapiro amplitudes.
Lecture II: In this lecture, I will mention KLT relations at tree level and broaden the view to study general properties of the amplitudes and some of their deeper structure in relation to modular forms.
14 December 2022, Brussels
Roberto Emparan (ICREA, Barcelona) | Holographic duals of black hole evaporation
I will discuss how the dynamical evaporation of a black hole through quantum Hawking emission admits a dual description in terms of classical gravitational evolution in Anti-de Sitter space. The idea was first proposed two decades ago, but it remained underdeveloped and poorly understood due to conceptual subtleties and technical difficulties. Recently, in collaboration with Luna, Suzuki, Tomasevic and Way (work to appear), we have applied novel methods to solve the dynamics of the holographic dual of black hole evaporation in a variety of interesting setups. The physical picture we obtain presents both satisfying and intriguing features.
Shahar Hadar (University of Haifa) | Fine structure of the photon ring
In the presence of a black hole, light sources connect to observers along multiple paths. Light rays that execute a number of orbits around the black hole appear near a closed critical curve in the observed image, giving rise to a sharp bright ring. This photon ring inherits a universal, intricate structure from the spacetime geometry and displays a self-similarity which governs the black hole’s near-critical perturbations. In the talk I will describe this emergent symmetry structure and the corresponding critical exponents. I will then discuss its consequences for observation by interferometric imaging and quasinormal gravitational-wave ringdowns. In particular, I will expand on a recent proposal to measure the ring with the next-generation Event Horizon Telescope by considering its fluctuations’ autocorrelations across different angles and times. Finally, I will discuss the symmetry’s constraints on any proposal for a holographic dual to the Kerr black hole.
7 December 2022, Leuven
Pietro Benetti Genolini (King’s College London) | Complex saddles and black holes in AdS_4
The large N limit of the superconformal index of ABJM theory is related to black holes that are asymptotically AdS_4. The relation can be made more precise by mapping the index to a supersymmetric partition function on a twisted S^1 x S^2 background, whose large N limit corresponds to the on-shell action of supersymmetric black hole solutions with appropriate boundary conditions. When computing the large N limit of the partition function, we encounter a family of complex saddles labelled by an integer. In this talk, I will consider the gravitational dual of these complex saddle points.
Kostas Skenderis (University of Southampton) | AdS/CFT @ loop order
I will discuss how to setup bulk renormalization in AdS and discuss the implications for AdS/CFT.
30 November 2022, Mons
Oliver Schlotterer (Uppsala University) | Modular graph forms and iterated integrals in string amplitudes
I will discuss string amplitudes as a laboratory for special functions and periods integrals that drive fruitful cross-talk with particle physicists and mathematicians. At genus zero, integration over punctures on a disk or sphere worldsheet generates multiple zeta values in the low-energy expansion of open- and closed-string amplitudes. At genus one, closed-string amplitudes introduce infinite families of non-holomorphic modular forms through the integration over torus puctures known as modular graph forms. The latter inspired Francis Brown’s alternative construction of non-holomorphic modular forms in the mathematics literature, and I will report on recent progress in clarifying their connection with modular graph forms.
Manus Visser (University of Cambridge) | Thermodynamic ensembles for de Sitter space and other causal diamonds
In 1977 Gibbons and Hawking (GH) famously derived the entropy of black hole and de Sitter (dS) horizons from a gravitational partition function. While their result is clearly correct, in the case of the dS horizons, the statistical foundation and interpretation of their approach has remained obscure, since there is no boundary in (Euclidean) dS at which to define the temperature or energy of the ensemble. Following recent work by Jacobson and Banihashemi I will explain how this issue can be addressed by introducing an artificial timelike boundary at which a thermodynamic ensemble can be defined, as has been done previously by York for black hole ensembles. In the limit where the boundary vanishes this provides a proper thermodynamic interpretation of the GH partition function, and I will argue it resolves a confusion about the minus sign in the first law of dS horizons. Moreover, I will show how the GH approach can be generalized to dS black holes and causal diamonds using the formalism of constrained instantons.
23 November 2022, Brussels
Fabio Apruzzi (University of Bern) | Non-invertible symmetries from holography and branes
Generalised non-invertible symmetries have emerged in past year as a new type of symmetry in d≥ 4 QFTs. This has lead to a new understanding of certain constraints on the low-energy dynamics of QFTs, such as QED, or the standard model. In this talk, I will describe a systematic approach to deriving symmetry generators of Quantum Field Theories in holography. Central to this are the Gauss law constraints in the Hamiltonian quantization of Symmetry Topological Field Theories (SymTFTs), which are obtained from supergravity. Crucially, we realize the symmetry generators from world-volume theories of D-branes in holography. I will particularly focus on non-invertible symmetries, by presenting the application of our proposal in the holographic setup, dual to 4d N=1 SU(M) or PSU(M) Super-Yang Mills (SYM). In the brane-picture, the fusion of non-invertible symmetries naturally arises from the Myers effect on D-branes. In addition, the action of the topological non-invertible defects t’Hooft on line is modelled by the Hanany-Witten effect. For SU and PSU SYM, we also identify the infra-red (IR) 4d field theory from the dual gravity description, which describes the domain-walls and confining/deconfining behaviour in the IR.
Marius Gerbershagen (VUB) | A gravity dual to computational complexity from first principles
Conjectured dualities between geometric features of AdS black hole geometries and computational complexity put forward by Susskind have raised interest in a holographic interpretation of distance measures in quantum circuits. In this talk, we discuss the derivation of such a duality from first principles. We consider quantum circuits for 2d CFTs given as a sequence of states related by conformal transformations and construct the time-dependent asymptotically AdS spacetimes dual to these circuits. Based on this construction, we map a canonical distance measure on Hilbert space – the Fubini-Study metric – to a geometric quantity in AdS using only the AdS/CFT dictionary as input. This allows us to construct the geometric dual to a computational complexity measure associated to this distance measure. With some caveats to be discussed in the talk, we reproduce the main expected features of computational complexity in states dual to black holes (linear growth and the switchback effect).
16 November 2022, Brussels
Shota Komatsu (CERN) | Large Charge ‘t Hooft Limit
N=4 super-Yang-Mills in four dimensions is integrable in the planar limit, allowing exact computations for a variety of observables. In this talk, I show that the large charge sector of N=4 SYM with the SU(2) gauge group provides another interesting solvable corner, which is far from the planar limit but nevertheless exhibits remarkably similar structures. I study non-BPS operators obtained by small deformations of half-BPS operators with large R-charge J, and analyze their spectrum in the double-scaling limit in which J is sent to infinity keeping the product of J and the gauge coupling fixed. I will show that the limit is controlled by the so-called centrally extended SU(2|2) symmetry, which played a crucial role in the integrability approaches in the planar limit.
Ioannis Tsiares (IPhT) | Universal OPE densities in (holographic) 2d CFTs
Any two-dimensional Conformal Field Theory (CFT) is defined via a list of primary operators, along with their scaling dimensions, spins, and OPE coefficients. This set of data, together with the central charge, uniquely specifies any correlation function of a given theory on an arbitrary Riemann surface. Using consistency conditions such as crossing symmetry and modular covariance, one can show that there are features of these data that are universal in all two-dimensional CFTs. A celebrated example of this sort is the Cardy formula for the asymptotic density of heavy states. In this talk, we will unravel an alternative method to obtain universal asymptotic expressions for the CFT data in any compact unitary two-dimensional CFT with central charge c>1. This method involves the study of the so-called ‘’Virasoro Crossing Kernels’’, and will allow us to obtain a Cardy-like formula for OPE densities of heavy primary operators, which is finite in the central charge. Along the way, we will discuss applications of our results in holography and highlight the importance of Crossing Kernels as a non-trivial kinematic tool in general in 2d CFTs, as well as in the construction of gravitational-looking densities of CFT data.
9 November 2022, Leuven
Dionysios Anninos (King’s College London) | Euclidean considerations on finite spacetimes
Andreas Blommaert (SISSA) | The powers of time
In finite entropy quantum systems, correlation functions do not eternally decay at late times, but instead fluctuate around a nonzero constant plateau. In gravity, those same correlation functions evaluated in black hole backgrounds do decay eternally. This apparent tension rooted in AdS/CFT raises the question of how the bulk gravitational description captures the late time plateau, thereby revealing its finite entropy nature. In this talk I will explain how the plateau can arise in the bulk from a perturbative sum (in the number of wormholes) over wormholes. In particular, the amplitude with g wormholes grows as time T to the power 2g+1, and this series over g converges to the plateau. I will discuss three independent derivations of this behavior. One is based on random matrices (this is essentially the boundary description in the sense of AdS/CFT). The second (which I will only briefly discuss) is based on Euclidean wormhole amplitudes. The third is based on new semiclassical Lorentzian spacetimes with topology change (wormholes). This talk will be based on work in progress with Jorrit Kruthoff and Shunyu Yao, and on a recent paper with the same folks.
26 October 2022, Mons
Ricardo Monteiro (Queen Mary University) | Celestial chiral algebras, colour-kinematics duality and integrability
We study celestial chiral algebras appearing in celestial holography, using the light-cone gauge formulation of self-dual Yang-Mills theory and self-dual gravity, and explore also their Moyal deformation, which is related to chiral higher-spin theories. The recently discussed w[1+∞] algebra in self-dual gravity arises from the soft expansion of an area-preserving diffeomorphism algebra, which plays the role of the kinematic algebra in the colour-kinematics duality and the double copy relation between the self-dual theories. The W[1+∞] deformation of w[1+∞] arises from a Moyal deformation of self-dual gravity. In all these theories, the chiral structure of the operator-product expansion exhibits the colour-kinematics duality: the implicit `left algebra’ is the self-dual kinematic algebra, while the `right algebra’ provides the structure constants of the operator-product expansion, ensuring its associativity. In a scattering amplitudes version of the Ward conjecture, the left algebra ensures the classical integrability of this type of theories. In particular, it enforces the vanishing of the tree-level amplitudes via the double copy.
Ashish Shukla (CPHT) | Bounds on gravitational brane couplings and tomography in AdS3 black hole geometries
In the talk, I will discuss AdS3 black hole geometries with an end-of-the-world (ETW) brane embedded in them, cutting off the second asymptotic region of the maximally extended spacetime. I will consider the ETW brane to have possible gravitational dynamics localized on it, the simplest possibility for which is to have JT gravity on the brane. Interestingly, as I will illustrate, bounds on the allowed rate of growth of entanglement in the CFT state dual to this geometry constrain the allowed values for the JT coupling on the brane. Also, by looking at the asymptotic growth of entanglement in the CFT state, one can read-off the brane parameters in the dual geometry, such as its location in the bulk and the value of the JT coupling, as I will elaborate upon in the talk.
19 October 2022, Brussels
Shlomo Razamat (Technion) | On IR dualities across dimensions
In this talk we will overview recent progress on understanding some aspects of IR dualities between Lagrangian constructions of 4d SCFTs and their engineering starting from 6d. We will illustrate these understandings mainly with the compactifications of D-type conformal matter and higher rank E-string theories down to four dimensions.
Eleni-Alexandra Kontou (Universiteit van Amsterdam) | Singularity theorems in semiclassical gravity
The classical singularity theorems predict the existence of singularities, defined using incomplete geodesics, under a set of general assumptions. One of those assumptions, namely the energy condition, is always violated by quantum fields and thus the realm of semiclassical gravity is outside the scope of these theorems. However, quantum fields do obey weaker conditions which can also be used to predict singularities. In this talk, I will present such semiclassical singularity theorems both in the timelike and the null case. Then I will argue for the need of singularity theorems with worldvolume averaged energy conditions and discuss the challenges and open questions for all cases.
12 October 2022, Leuven
Linus Wulff (Masaryk University) | T-duality and alpha’-corrections
I will report on some recent progress in using T-duality to constrain the form of alpha’-corrections to the string effective action. We will see that for the heterotic string the first two corrections can be described using a tool known as Double Field Theory (DFT), while this is not the case for Type II strings.
Christian Copetti (SISSA Trieste) | Non-Invertible Duality Defects and Holography
We review how non-invertible self duality symmetries can arise in dimension d>2. We then explain how these structures appear in theories with an holographic dual through emergent discrete gauge symmetries. We study the example of N=4 SYM with gauge group SU(N) and (time permitting) comment on generalizations to class S.
5 October 2022, Brussels
Renann Lipinski Jusinskas (Prague, Inst. Phys.) | Higher spin theories from the worldsheet
In this talk I will discuss chiral (twisted) strings, which appeared as a tensionful generalization of the ambitwistor string. Their physical spectrum matches the usual closed string massless spectrum plus the first massive level of the open string. More recently, a new class of twisted strings was introduced, in which the massive spectrum corresponds to a single mass level (N) of the open string. Naturally, this gives us access to a higher spin field theory coupled to gauge theory and gravity. I will try to avoid more technical discussions, and focus on the more general properties of these models. In particular I would like to speculate about their field theory realization and its interface with string theory.
João Caetano (CERN) | Unoriented Quantum Field Theories: from crosscaps to holography
In two dimensions, one can study quantum field theories on unoriented manifolds by introducing crosscaps.
This defines a class of states called crosscap states which share a few similarities with the notion of boundary states.
In this talk, I will show that integrable theories remain integrable in the presence of crosscaps, and this allows to exactly determine the crosscap state. In four dimensions, the analog is to place the quantum field theory on the real projective space, the simplest unoriented 4-manifold. I will show how to do this in the example of N=4 Supersymmetric Yang-Mills, discuss its holographic description and present a new solvable setup of AdS/CFT.