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Seminars at

Found at least 20 result(s)

17.10.2023 (Tuesday)

Z_N duality and parafermions revisited

Exceptional Seminar Zhihao Duan (QMUL)

at:
11:00 QMUL
room GO Jones 610
abstract:

The Symmetry Topological field theory (SymTFT) has played an important role in recent studies of symmetries in physics. As one application, it provides a clear and unifying picture of bosonization and fermionization in two dimensions. On the other hand, for theories having a non-anomalous Z_N symmetry with N > 2, people have long speculated about the presence of parafermions. In this informal talk, we revisit the bosonization/parafermionization procedure from the SymTFT point of view, and explain some subtleties and peculiarities involved. It will be based on 2309.01913 and (hopefully) accessible to a broad audience. Part of the London TQFT journal club (https://www.london-tqft.co.uk)

12.10.2023 (Thursday)

Torus 1-point functions for the su2 WZW model and their modular properties

Regular Seminar Simon Wood (Cardiff U.)

at:
14:00 QMUL
room G.O. Jones 610
abstract:

Characters of representations of chiral algebras are important tools in conformal field theory. They are a special chase of chiral torus 1-point functions (namely those where the vacuum has been inserted) and their modular properties famously give rise to the Verlinde formula. In this talk we will generalise from vacuum insertions to insertions from any irreducible representation in the example of the su2 WZW models at non-negative integral level and explore their modular properties.

10.10.2023 (Tuesday)

Black hole from matrices for dummies

Regular Seminar Masanori Hanada (QMUL)

at:
14:00 QMUL
room MB-503
abstract:

We will give an intuitive explanation of why and how matrices (or, more precisely, large-N gauge theories) can describe a black hole, without assuming knowledge of quantum mechanics and holographic duality. Firstly, we explain an intuitive picture inroduced by Witten: diagonal entries of matrices describe particles and off-diagonal entries describe strings connecting particles. When many strings are excited, a lot of energy and entropy are packed in a small region and form black hole. Next, we consider classical dynamics of matrix model. Specifically, we colide two black holes. Using the energy conservation, equipartition law of energy and elementary school math, we show that black hole becomes colder after the merger. Matrices know black hole's negative heat capacity! To gain a little bit more intuition, we will look at ants. Collective behavior of ants has a striking similarity to black hole. The mapping rule is ant -> particle, pheromone -> string, and ant trail -> black hole. Tuning parameters such as temperature or each ant's laziness, we can obtain three kinds of phase diagrams. Each of them has a counterpart in large-N gauge theories. If time permits, I will explain the mechanism applicable to strongly-coupled and highly quantum regime needed for quantitative agreement with Einstein gravity. (This part requires a good understanding of undergraduate-level quantum mechanics.)

05.10.2023 (Thursday)

Amplitudes in the sky

Regular Seminar Scott Melville (Queen Mary U. of London)

at:
14:00 QMUL
room G.O. Jones 610
abstract:

I will describe some recent developments in applying amplitude and effective field theory ideas in cosmology, and focus in particular on what cosmological measurements (of e.g. the cosmic microwave background or large-scale structure of galaxies) can tell us about the underlying fundamental field theory.

03.10.2023 (Tuesday)

Renormalization group flows in AdS and the bootstrap program

Regular Seminar Marco Meineri (Turin U.)

at:
14:00 QMUL
room Maths MB-503
abstract:

We consider the renormalization group flow of a quantum field theory (QFT) in Anti-de Sitter (AdS) space. We derive sum rules that express UV data and the energy of a chosen eigenstate in terms of the spectral densities and certain correlation functions of the theory. In two dimensions, this leads to a bootstrap setup that involves the UV central charge and may allow us to follow a Renormalization Group (RG) flow non-perturbatively by continuously varying the AdS radius. Along the way, we establish the convergence properties of the newly discovered local block decomposition, which applies to three-point functions involving one bulk and two boundary operators.

28.09.2023 (Thursday)

Kerr Amplitudes from Higher Spin Theory

Regular Seminar Lucile Cangemi (Uppsala University)

at:
14:00 QMUL
room G.O. Jones 610
abstract:

Higher-spin theory and massive gauge invariance can be used as input for constraining root-Kerr and Kerr amplitudes, relevant for calculating gravitational observables with spin. Elegant three-point spin-s amplitudes exist for Kerr black holes, however constructing the corresponding four-point Compton amplitudes is an open problem. In this talk, I will discuss the origin of the Kerr three-point amplitudes from a higher-spin theory perspective. Guided by higher-spin constraints and classical-limit analysis, I will propose quantum and classical tree-level Compton amplitudes relevant for root-Kerr and Kerr to all orders in spin.

21.09.2023 (Thursday)

Bootstrability with Improved Truncation Methods

Regular Seminar Vasilis Niarchos (Crete U.)

at:
14:00 QMUL
room G.O. Jones 610
abstract:

I will discuss recent work on the numerical conformal bootstrap that modifies the scope of the so-called truncation methods and improves their efficiency. I will show how the proposed approach performs in the recently discussed context of bootstrability for 1d defect CFTs on 1/2 BPS Wilson lines in 4d N=4 SYM theory, how it compares with the more standard linear functional method and how different algorithms allow us to tackle the large-scale non-convex optimization problems that are involved in this method. Along the way, I will address the prospects of AI and Machine Learning in this particular direction.

14.06.2023 (Wednesday)

Holographic description of code CFTs

Exceptional Seminar Anatoly Dymarsky (U Kentucky)

at:
12:00 QMUL
room G.O. Jones 610
abstract:

Recently, a relation was introduced connecting codes of various types with the space of abelian (Narain) 2d CFTs. We extend this relation to provide holographic description of code CFTs in terms of abelian Chern-Simons theory in the bulk. For codes over the alphabet Z_p corresponding bulk theory is, schematically, U(1)_p times U(1)_{-p} where p stands for the level. Furthermore, CFT partition function averaged over all code theories for the codes of a given type is holographically given by the Chern-Simons partition function summed over all possible 3d geometries. This provides an explicit and controllable example of holographic correspondence where a finite ensemble of CFTs is dual to "topological/CS gravity" in the bulk. The parameter p controls the size of the ensemble and "how topological" the bulk theory is. Say, for p=1 any given Narain CFT is described holographically in terms of U(1)_1^n times U(1)_{-1}^n Chern-Simons, which does not distinguish between different 3d geometries (and hence can be evaluated on any of them). When p approaches infinity, the ensemble of code theories covers the whole Narain moduli space with the bulk theory becoming "U(1)-gravity" proposed by Maloney-Witten and Afkhami-Jeddi et al.

30.05.2023 (Tuesday)

Gluon scattering in AdS at finite string coupling

Regular Seminar Shai Chester (Harvard)

at:
15:30 QMUL
room G O Jones 610
abstract:

We consider gluons scattering in Type IIB string theory on AdS5 x S^5/Z2 in the presence of D7 branes, which is dual to the flavor multiplet correlator in a certain 4d N=2 USp(2N) gauge theory with SO(8) flavor symmetry. We compute this holographic correlator in the large N and finite string coupling tau expansion using constraints from derivatives of the mass deformed sphere free energy, which we compute to all orders in 1/N and finite tau using localization. In particular, we fix the F^4 correction to gluon scattering on AdS in terms of Jacobi theta functions, and the D^2F^4 correction in terms of a non-holomorphic Eisenstein series. At weak string coupling, we find that the AdS correlator takes a remarkably similar form as the flat space Veneziano amplitude. Finally, we combine the numerical conformal bootstrap with the localization constraints to study the correlator at finite N and tau.

04.05.2023 (Thursday)

Self-dual gravity and color/kinematics duality in AdS4

Regular Seminar Silvia Nagy (Durham)

at:
14:00 QMUL
room 610
abstract:

I will show that self-dual gravity in Euclidean four-dimensional Anti-de Sitter space (AdS4 ) can be described by a minimally coupled scalar field with a cubic interaction written in terms of a deformed Poisson bracket, providing a remarkably simple generalisation of the Plebanski action for self-dual gravity in flat space. This implies a novel symmetry algebra in self-dual gravity, notably an AdS4 version of the so-called kinematic algebra. This provides a concrete starting point for defining the double copy for Einstein gravity in AdS4 by expanding around the self-dual sector. Moreover, I will show that the new kinematic Lie algebra can be lifted to a deformed version of the w1+∞ algebra, which plays a prominent role in celestial holography.

02.05.2023 (Tuesday)

Small Bosonic CFTs, Chiral Fermionization, and Symmetry/Subalgebra Duality

Exceptional Seminar Brandon Rayhaun (Simons Center for Geometry and Physics)

at:
15:30 QMUL
room GO Jones 610
abstract:

Conformal field theories in (1+1)D are key actors in many dramas in physics and mathematics. Their classification has therefore been an important and long-standing problem. In this talk, I will describe the main ideas behind the classification of (most) "small" bosonic CFTs - theories with low central charge (less than 24) and few primary operators (less than 5). I will then highlight two applications of this result. First, I will describe how it can be used in tandem with bosonization and fermionization techniques to establish the classification of chiral fermionic CFTs with central charge less than 23. Second, I will showcase how it can be used to bootstrap the generalized global symmetries of chiral bosonic CFTs. Talk based on arXiv:2208.05486 [hep-th] (joint work with Sunil Mukhi) and arXiv:2303.16921 [hep-th].

25.04.2023 (Tuesday)

Generalised symmetries and dualities on the lattice

Exceptional Seminar Clement Delcamp (Gent U)

at:
15:30 QMUL
room GO Jones 610
abstract:

Generalised notions of symmetries have received widespread attention in recent years. Though exotic, such generalised symmetries have been shown to naturally arise for instance as dual symmetries upon gauging ordinary symmetries. In this talk, I will present a systematic framework to investigate dualities of quantum lattice models and study the resulting generalised symmetries. I will illustrate this framework in the context of familiar spin models.

20.04.2023 (Thursday)

Evanescent integrals from local subtraction

Regular Seminar Alessandro Georgoudis (Nordita)

at:
14:00 QMUL
room 610
abstract:

When computing scattering amplitudes in dimensional regularization, one frequently encounters contributions whose integrands vanish in strictly four dimensions. While these "evanescent" integrals can be handled with dimensional shift identities at one-loop, a similar treatment at the next perturbative order is insufficient. In this talk, we introduce a novel systematic method to compute evanescent contributions. By employing the local subtraction method of Anastasiou and Sterman we show that evanescent Feynman integrals are controlled by regions of loop-momentum space associated to ultra-violet, soft or collinear divergences. These integrals are then reduced to either products of one-loop integrals or one-fold integrals thereof. Starting from known integrands, we use this technique to easily recompute the leading-color two-loop four- and five-gluon QCD amplitudes in the all-plus helicity configuration. Remarkably, we find that the finite remainder is given by contributions arising from only ultra-violet regions of momentum space, and that the collinear contributions cancel in a highly non-trivial way.

13.04.2023 (Thursday)

IR dualities across dimensions

Regular Seminar Shlomo Razamat (Technion)

at:
14:00 QMUL
room 610
abstract:

We will overview some of the recent progress regarding IR dualities across dimensions. In such dualities one engineers the same low energy QFT starting from high energy descriptions in different space-time dimensions. We will review how various strong coupling phenomena can follow from embedding the system of interest in such an across dimensions setup. We will also outline some of the open problems/questions in the field.

12.04.2023 (Wednesday)

Lieb-Schultz-Mattis, t Hooft and Luttinger: anomalies in lattice systems

Triangular Seminar Meng Cheng (Yale)

at:
15:00 QMUL
room David Sizer Lecture Theatre
abstract:

In this talk I will discuss aspects of global symmetry and their t Hooft anomalies in lattice systems. I will discuss how anomalies are defined and probed using topological defects, which will be applied to both internal and lattice symmetries. The latter arise in systems satisfying Lieb-Schultz-Mattis-type theorems. Using the example of a spin-1/2 XXZ chain, I will also discuss how the continuum limit of a lattice model is properly described in terms of a low-energy theory with topological defects. In particular, I will show that t Hooft anomaly explains a curious size dependence of the ground state lattice momentum in the spin-1/2 XXZ chain.

12.04.2023 (Wednesday)

String duals of two-dimensional Yang-Mills and symmetric product orbifolds

Triangular Seminar Shota Komatsu (CERN)

at:
16:30 QMUL
room David Sizer Lecture Theatre
abstract:

We propose a (bosonic) worldsheet description of two-dimensional Yang-Mills. We also argue that similar worldsheet actions provide candidate duals to the symmetric product orbifolds for arbitrary seed CFTs.

30.03.2023 (Thursday)

A CFT perspective on AdS amplitudes

Regular Seminar Agnese Bissi (ICTP)

at:
14:00 QMUL
room zoom
abstract:

In this talk I will discuss how to compute amplitudes on AdS, using the analytic conformal bootstrap and the AdS/CFT correspondence. I will also discuss how to include higher trace operators in a simplified setup.

23.03.2023 (Thursday)

Log-CFTs, Skyrmions, and Quivers from 3d N=2 theories

Regular Seminar Sergei Gukov (DIAS)

at:
14:00 QMUL
room G. O. Jones 610
abstract:

In this talk we will explore a class of 3d N=2 theories labeled by graphs and related to quivers in an unusual way. Unlike quiver gauge theories --- a class of Lagrangian field theories widely used in modern QFT --- theories that we consider are non-Lagrangian, in a sense that they can be defined as IR fixed points of gauge fields coupled to non-linear matter as in the Skyrme models of nuclei. Just like the physics of the Skyrme model is intimately tied to symmetries of QCD, generalized symmetries play an important role in these 3d N=2 theories. The connection to quivers, on the other hand, arises in a way that is not standard in modern particle physics, but is standard in the study of logarithmic CFTs and motivic DT invariants. Download Mathematica demonstration here: theory.caltech.edu/~gukov/Plumbed.nb 3d Modularity software: https://github.com/d-passaro/pySeifert

22.03.2023 (Wednesday)

On anomalies and gauging of U(1) non-invertible symmetries in 4d QED

Exceptional Seminar Avner Karasik (Cambridge University)

at:
12:00 QMUL
room GO Jones 610
abstract:

Abstract: I will present a way to promote the anomalous axial U(1) in 4d QED to an exact symmetry, with the price of losing its invertibility. I will then discuss some applications of this non-invertible U(1) symmetry. In particular, I will show how to couple this non-invertible symmetry to a gauge field. By taking this gauge field to be dynamical, we get a new type of gauge theory with unconventional interactions and constraints. By taking this gauge field to be background, we can study 't-Hooft anomalies of the non-invertible symmetry.

16.03.2023 (Thursday)

Classification of Superconformal field theories in four dimensions

Regular Seminar Mario Martone (KCL)

at:
14:00 QMUL
room G. O. Jones 610.
abstract:

In this talk I will give you an update of the status of the classification of superconformal field theories in four dimensions. After reviewing the basic properties which make the classification of theories with both supersymmetry and conformal invariance possible, I will describe in detail the framework which allows for a bottom up analysis and summarise the latest results in rank-2.