2024 SEUYC Theoretical Physics Seminars -- Upcoming

8th November, 2024

16:00-17:00

Christoph Dlapa

(DESY)

Abstract

Feynman integrals are important for nearly every computation in perturbative high-energy physics. They are required in order to extract predictions from the theory beyond the leading perturbative order. As a consequence, they are important in many areas, e.g. collider physics, gravitational waves, the evolution of the universe, lattice gauge theory, string theory, and pure mathematics. In this talk, I will discuss a highly efficient machinery for computing state-of-the-art Feynman integrals. This includes integration-by-parts reduction, (canonical) differential equations, the method of regions, and the Feynman parameter method. All techniques are introduced pedagogically through simple examples.

25th October, 2024

16:00-17:00

Pietro Longhi

q-difference WKB Analysis and q-Painleve’ Equations

Pietro2024.mp4

Abstract

In this talk I will present an approach to the study of second order q-difference equations based on the WKB approximation, and comment on implications for broader correspondences with q-Painleve’ equations and wall-crossing in five-dimensional supersymmetric Quantum Field Theory.

18th October, 2024

16:00-17:00

Ziwen Kong

(DESY)

Abstract

I will present arXiv: 2203.17157 with N. Drukker and G. Sakkas and the paper to appear with N. Drukker and P. Kravchuk. Symmetry-breaking is an innate nature of defects. There is a distinguished set of defect operators that keeps track of the symmetries in the parent conformal field theory broken by the defect insertion, such as the tilt operators and displacement operators. We find identities of such defect operators between their 2-pt functions and integrated 4-pt functions, based on either the geometric properties of the defect conformal manifold which is the symmetry-breaking coset, or the Lie algebra of corresponding broken symmetry generators. I will demonstrate these integral identities in the case of the 1/2 BPS Maldacena-Wilson loop in N = 4 SYM as an example.

11th October, 2024

11:00-12:00

Connor Behan

(ICTP-SAIFR)

Abstract

Realistic experimental systems often contain boundaries, interfaces and impurities which can be modelled as nonlocal operators called defects. The infrared behaviour of them can be studied using conformal field theory techniques but many technical challenges arise when implementing the conformal bootstrap for defects. I will discuss defects for a free CFT which is a non-trivial case that still allows much progress to be made. In analogy with supersymmetric CFT, these theories contain a protected sector which can be solved exactly and an unprotected sector where numerics offer the most concrete results. I will discuss progress that has been made in both of these areas over the last few years.

27th September, 2024

16:00-17:15

Jie Ren

(Sun Yat-sen University)

Abstract

In the context of the AdS/CFT correspondence, we aim to construct AdS spacetimes with reduced symmetry to study more general states in the CFT. When the geometry depends on a spatial coordinate in addition to the AdS radial coordinate, we typically need to solve Einstein's equations numerically. I will introduce numerical relativity in AdS/CFT, focusing on the theoretical aspects of stationary solutions to Einstein's equations. In AdS spacetimes, black hole solutions evade the no-hair, topology, and rigidity theorems applicable to asymptotically flat spacetimes, leading to a significantly richer variety of solutions. Topics include ellipticity, gauge-fixing methods, boundary conditions, and numerical algorithms. Physics examples include black funnels, droplets, tunnels, hammocks, and holographic quantum matter.

20 September, 2024

Jules Lamers

（IPhT, CEA-Saclay,  France）

Abstract

Spin chain are quantum-mechanical models for magnetic materials. Special examples are (quantum) integrable: they have many conserved charges whose spectrum can be determined exactly thanks to a rich underlying algebraic structure. While one traditionally assumes that only neighbouring spins interact, there are also integrable spin chains with long-range interactions. Their integrability hinges on connections to quantum many-body systems of Calogero--Sutherland type. This is by now rather well understood in the (truly long-range) trigonometric case, but much less so in the elliptic (intermediate-range) case.

In this talk I will give an overview of the key results for integrable long-range chains, and outline how integrability works for these models.

Based on recent and ongoing work with R Klabbers (Humboldt U Berlin) and D Serban (IPhT).

June 18th (Tues.), 2024

Jun Nian

(University of Chinese Academy of Sciences)

Abstract

Black holes are fascinating objects in nature. Although they are introduced as classical solutions in general relativity, their intrinsic nature should be quantum, which manifests during the black hole formation and evaporation processes. If string theory is claimed to be a quantum gravity candidate, it should be able to provide a consistent picture and elucidate some perspectives for black hole formation and evaporation. In this talk, we will focus on rotating Kerr black holes. On the one hand, we will show through a toy model that Kerr black holes can emerge naturally from the Virasoro minimal string theory. On the other hand, we use a field-theoretic approach to compute the time evolution of entanglement entropy between an evaporating Kerr black hole and its Hawking quanta, i.e., the Page curve, which provides a new resolution to the long-standing black hole information paradox. This talk is based on my recent papers, 2312.14287 and 2210.06762, and some work in progress.

May 21st (Tues.), 2024

Hjalte Frellesvig

(Niels Bohr Institute, Denmark)

Abstract

Feynman integrals are the main mathematical building block of high precision contributions to scattering amplitudes in quantum field theories.

May 14th (Tues.), 2024

Scott Collier

 (MIT)

3d quantum gravity and Virasoro TQFT

Slides.pdf

Abstract

I will discuss the quantization of one of the simplest theories of gravity, three-dimensional Einstein gravity with negative cosmological constant. In particular I will describe a precise reformulation of AdS_3 quantum gravity in terms of a novel topological quantum field theory based on the quantization of the Teichmuller space of Riemann surfaces that we refer to as Virasoro TQFT. This proposal clarifies the relationship between pure AdS_3 quantum gravity and (two copies of) SL(2,R) Chern-Simons theory, and resolves some well-known issues with this lore. I will argue that moreover this proposal provides a practical framework for the computation of 3d gravity partition functions, and will describe how it elucidates various aspects of the holographic dual.

Apr. 30th (Tues.), 2024

Fabio Apruzzi

(University of Padua, INFN, Italy)

Abstract

Recently it has been observed that the string theory branes in geometric engineering and holography have a striking connection with generalized global symmetries. I will explain how branes, in a certain topological limit, not only furnish the symmetry generators, but also encode the so-called Symmetry Topological Field Theory (or SymTFT). For a d-dimensional quantum field theory (QFT), this is a (d+1)-dimensional topological field theory, whose topological defects encode both the symmetry generators (invertible or non-invertible) and the generalized charges. After having introduced the general setup, I will exemplify this for QFT with finite generalized (non-)invertible symmetries. Finally, I will discuss this prescription to non-finite symmetries. In particular, I will discuss the symmetry theory (SymTh) for non-invertible symmetries with transformation parameters in Q/Z, which are the ones that also appear in 4d particle physics. 

Apr. 23rd (Tues.), 2024

Congkao Wen

Abstract

In this talk, I will discuss some exact results of four-point correlation functions in N=4 super Yang-Mills. I will focus on the correlator involving two determinant operators (often called giant gravitons) and two superconformal primary operators in the stress tensor multiplet, and study the associated integrated correlator by integrating over the spacetime dependence. I will show that the integrated correlator can be reformulated as an infinite sum of protected three-point functions, which leads to exact expressions of the integrated correlator in the planar limit and beyond. The SL(2, Z) completion of the results and the comparison with string theory will be discussed as well.

Apr. 16th (Tues.), 2024

Carlos Nunez 

(Swansea University)

Abstract

I will present recent advances in the duality between gauge fields and strings. In particular I will focus on holographic dual to field theories with a UV-conformal point and confining IR dynamics with large number of colours and flavours. The presentation is planned to be pedagogical, avoiding many technical details, trying to convey the main idea. The talk is based on the papers I published in the last five months and work in progress

Apr. 11th (Thurs.), 2024

Rongxin Miao

(Sun Yat-sen University)

Abstract

This talk discusses the Casimir effect of a wedge and its holographic dual. We prove that the wedge Casimir effect is universally determined by the displacement operator in smooth limit. Besides, we argue that the wedge Casimir energy increases with the opening angle. Furthermore, we construct the holographic dual of wedges in AdS/BCFT in general dimensions. We verify that our proposal can produce the correct smooth limit. Next, we discuss the wedge contribution to holographic entanglement entropy and find it increases with the opening angle, similar to the wedge Casimir energy. Finally, we conclude with some open questions.

Apr. 9th (Tues.), 2024

Shota Komatsu

(CERN, Switzerland)

Abstract

We study analytical properties and integrable structures of the meson spectrum in large Nc QCD2. We show that the integral equation that determines the masses of the mesons, often called the ’t Hooft equation, is equivalent to finding solutions to a TQ-Baxter equation. Using the Baxter equation, we extract systematic expansions of the energy levels as well as analytic asymptotic expressions for wavefunctions. Our analysis extends previous results for a special quark mass by Fateev et al. to arbitrary quark masses. This reformulation, together with its relation to an inhomogeneous Fredholm equation, is particularly suited for analytical treatments and makes accessible the analytic structure of the spectrum in the complex plane of the quark masses. We also  comment on applications of our techniques to non-perturbative topological string partition functions.

Mar. 26th (Tues.), 2024

Abstract

This presentation focuses on the lattice and matrix bootstrap methods, distinguished by their utilization of the equation of motion as bootstrap constraints. These methods share key characteristics with the closely related fields of quantum mechanics bootstrap and many-body bootstrap. The presentation will cover the application of the bootstrap method to the matrix model theory, with a particular emphasis on the ground state of BFSS. Additionally, I will discuss the latest results in bootstrap finite N matrix model theory, including the upcoming bootstrap result of SU(2) lattice gauge theory.

Mar. 21st (Thur.), 2024

Bartek Czech

(IAS, Tsinghua University)

Abstract

An important class of quantum states are those whose entanglement entropies can be computed by minimal cuts through some bulk structure---a holographic spacetime or a random tensor network. Such states obey linear constraints on their entanglement entropies, which are known as holographic entropy inequalities. I present two new infinite families of holographic entropy inequalities. The entropies featured in these inequalities are best visualized on graphs whose incidence relations reflect subsystem inclusion. These graphs turn out to be tessellations of the torus and the real projective plane. The non-contractible cycles on these manifolds play an indispensable role in proving the inequalities, which shows that they hold for essentially topological reasons. Physically, the inequalities represent constraints on holographic error correcting codes and, in some cases, the entropy of two-sided black holes. If time allows it, I will sketch bonus applications, which involve a non-planar polytope called cubohemioctahedron and (more speculatively) the toric code.

Mar. 19th (Tues.), 2024

Sameer Murthy

(King's College London, UK)

Abstract

The fundamental heterotic string has a tower of BPS states with an exponential growth in the charges. The fate of these BPS states at strong coupling is an old, much-debated topic: do they become a black hole or a string gas? I will discuss a new approach to this problem, i.e. the gravitational path integral corresponding to the supersymmetric index of these states. I will show that the saddle-point configuration of this path integral is a supersymmetric rotating non-extremal Euclidean black hole. This configuration is singular in the two-derivative theory but is resolved by higher-derivative terms from string theory. Remarkably, the one-loop, four-derivative F-term contribution to the prepotential leads to a precise match of the gravitational and microscopic index. Thus, the nature of the BPS string at strong coupling depends on the precise observable being probed. The thermal states transition to a winding condensate and a gas of strings without ever reaching a small black hole, while the index is captured by the rotating Euclidean black hole solution, is constant, and thus smoothly connected to the microscopic ensemble.

Mar. 12th (Tues.), 2024

Alexey S. Koshelev

(ShanghaiTech University)

Abstract

In my talk I will explain how infinite derivatives naturally arise in consideration of quantum properties of general gravity theories. I will construct the most general propagator around Minkowski and (anti)-de Sitter backgrounds proving that an infinite tower of derivatives is inevitable in order to exorcise ghosts. Then I will show that this construction has a direct relation to strings, especially to string field theory. I will elaborate on the issue of ghosts and on a connection with a recent idea of stubs in QFT and SFT.

Mar. 5th (Tues.), 2024

16:00-17:00

Bethe ansatz in 2d conformal field theory

Slides.pdf

Abstract

The usual approach to study 2d CFT relies on the Virasoro algebra and its representation theory. Moving away from the criticality, this infinite dimensional symmetry is lost so it is useful to have a look at 2d CFTs from more general framework of quantum integrability. Every 2d conformal field theory has a natural infinite dimensional family of commuting higher spin Hamiltonians that can be constructed out of Virasoro generators. Perhaps surprisingly two different sets of Bethe ansatz equations are known that be used to diagonalize these (one by Bazhanov-Lukyanov-Zamolodchikov and another by Litvinov). I want to discuss these constructions as well as their relation to W algebras and the affine Yangian symmetry.

Feb. 27th (Tues.), 2024

16:00-17:00

Matthias Gaberdiel

Abstract

The symmetric orbifold of T^4, Sym_N(T4), is dual to tensionless string theory on AdS3 x S3 x T4 with one unit of NS-NS flux. In this talk I will study the deformation of the symmetric orbifold theory that corresponds to switching on R-R flux (and hence the string tension) in the bulk string theory. Working in the planar large N limit as well as the limit of large charge, I will explain how the deformed symmetric orbifold theory can be explicitly solved in terms of what appears to be an integrable system. Among other things, this allows us to give explicit predictions about the anomalous conformal dimensions of all multi-magnon states.

Jan. 16th (Tues.), 2024

16:00-17:00

(CERN, Switzerland)

Abstract

Lorentzian correlators of local operators exhibit surprising singularities in theories with gravity duals. These are associated with null geodesics in an emergent bulk geometry. I will talk about singularities of the thermal response function dual to propagation of waves on the AdS Schwarzschild black hole background. I will present the analytic form of the leading singularity dual to a bulk geodesic that winds around the black hole. Remarkably, it exhibits a boundary group velocity larger than the speed of light, whose dual is the angular velocity of null geodesics at the photon sphere. The strength of this singularity is controlled by the classical Lyapunov exponent associated with the instability of nearly bound photon orbits.