UEICMSSeminar Room
FIRST seminar
SECOND seminar
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ICMSLecture Theatre (room 5.10)2018-10-03Preseminar for PhD studentsAmihay HananyImperial CollegeDiscrete Gauging in six dimensionsWhen M5 branes are placed in an A type ALE background there are tensionless strings which arise each time 2 or more M5 branes coincide. The system has no low energy Lagrangian description and new techniques need to be applied in order to figure out the relevant physics of such coincident M5 branes. The world volume theory on the M5 branes has (0,1) supersymmetry in 6 dimensions with massless vector, tensor and hyper multiplets. There are two branches on the moduli space of vacuum configurations where scalar fields in tensor and hyper multiplets receive VEVs, respectively. In this talk we will focus on the Higgs branch of the low energy theory and find new and fairly surprising results. Each time there is a new tensionless string, there is a new Higgs branch, thus the theory has a multitude of Higgs branches depending on the types of tensionless strings in the spectrum. There are two main effects: “discrete gauging” and “small instanton transitions” (when one extends beyond A type) and a need to efficiently describe the so many Higgs branches which arise. The main tool is the Coulomb branch of 3d N=4 gauge theories, which has been studied intensively by many of my collaborators.
Bart VlaarHeriot-WattThe reflection equation: history, concepts and solutionsQuantum integrability is typically underpinned by factorizability criteria (say, of a scattering process). Away from boundaries this is the Yang-Baxter equation, a cubic relation which is intimately connected to certain bialgebras known as affine quantum groups and their representations. In the presence of a boundary, a quartic relation known as the reflection equation is the natural analogue. It arises naturally in the representation theory of certain pairs (affine quantum group, coideal subalgebra). We will survey this and highlight some recent work: the classification of solutions in the case of quantum affine sl_N (joint work with V. Regelskis). We will also outline some work in progress about the axiomatic characterization and construction of so-called universal solutions (joint with R. Weston and A. Appel, respectively).
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ICMSLecture Theatre (room 5.10)2018-10-17Preseminar for PhD studentsJan de BoerUniversity of AmsterdamProbing the interior of typical black hole microstatesTBA
Marc-Antoine FisetUniversity of OxfordSuperconformal algebras and G-structure symmetries in generic (1,0) sigma-modelsConformal field theories with higher spin currents have been known to govern type II string dynamics in exceptional holonomy manifolds since the work of Shatashvili and Vafa. From a nonlinear sigma-model viewpoint, the reduction of holonomy yields chiral symmetries extending (1,1) supersymmetry. I will generalise this result to generic (1,0) models with Fermi superfields, appropriate in particular for heterotic compactifications. I will then comment on anomalies of these symmetries, as a first step to bridge the gap between target space geometry and abstract conformal field theory. Based on 1809.01138.
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ICMSLecture Theatre2018-10-31Preseminar for PhD studentsDavid SkinnerCambridgeAmbitwistor Strings in AdSAmbitwistor strings are chiral worldsheet theories that correspond to a field theory on their target space. On flat space, they provide the theoretical framework in which to understand the `scattering equations’ approach to amplitudes in massless field theories pioneered by Cachazo, He and Yuan. These strings are consistent whenever the background space-time obeys Einstein’s equations, so in principle can also be used to resum supergravity Witten diagrams in AdS. I report on progress in this direction, seeking an explicit form for the `AdS scattering equations’.
Paolo GloriosoUniversity of ChicagoEffective field theory and topological effective response away from equilibriumIn the first part of the talk I will introduce the formalism of non-equilibrium EFT and show how it provides a unified field theory description of the low-energy behavior of systems in local thermal equilibrium. I will then describe recent progress showing consistency with unitarity and causality at (perturbative) quantum level. In the second part of the talk I will discuss effective theory for Floquet systems, which do not possess any strict notion of equilibrium. I will show how one can adapt the general approach of non-equilibrium EFT to describe topological response of such systems.
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ICMSSeminar Room2018-11-14Preseminar for PhD studentsDavid VeghQueen MaryThe goldfish and the stringThe classical motion of a string in AdS_3 spacetime is governed by the generalized sinh-Gordon theory. This theory has singular soliton solutions which correspond to cusps on the string. In this talk, I examine string solutions where such cusps are classically pair-produced. For the calculations, I use an exact discretization of the equation of motion. If time permits, I will also discuss some recent results.
Pietro Benetti GenoliniCambridgeTopological AdS/CFT and gravitational free energyI will describe holographic duals to topologically twisted gauge theories on Riemannian four- and three-manifolds. Imposing boundary conditions for the supergravity fields corresponding to the topological twists, the renormalised supergravity action is independent of the choice of boundary metric. Under additional regularity assumptions, the on-shell action can be evaluated and shown to always vanish, which provides a prediction for the large N limit of topologically twisted theories.
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ICMSLecture Theatre2018-11-28Preseminar for PhD students
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Marius de LeeuwTrinity College DublinCorrelation functions in AdS/dCFTIn this talk I will consider N=4 SYM in the presence of a codimension 1 defect. This defect CFT is dual to a certain probe D3-D5 brane set-up. I will introduce a framework for quantum computations in this non-trivial dCFT and use it to compute one- and two-point functions. I will discuss the tree-level and one-loop computations to the one-point function of the BPS vacuum state, finding perfect agreement with an earlier string-theory prediction. Second I will discuss two-point functions and their relation to the boundary conformal bootstrap equations and three-point functions in N=4 SYM.
Agata SmoktunowiczUniversity of EdinburghOn set-theoretic solutions of the quantum Yang-Baxter equation, nilpotent rings and the Reflection EquationIn 2005, Wolfgang Rump discovered some connections between noncommutative ring theory and set theoretic solutions of the quantum Yang-Baxter equation. In particular, he showed that every nilpotent ring gives a highly non-trivial solution to the Yang-Baxter equation.
In the first part of this talk we will present the introduction to this research area and explain how to obtain set-theoretic solutions from nilpotent rings, using Rump's theorems. In the second part we will present new solutions of the Reflection Equation. Our solutions work for both set-theoretic and parameter dependent solutions of the Reflection Equation and are obtained from nilpotent rings.
This is a joint work with with Robert Weston and Leandro Vendramin.
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ICMSLecture Theatre2018-12-12Preseminar for PhD studentsBenjamin DoyonKing's College LondonHydrodynamics of integrable systems, and applications to non-equilibrium transportHydrodynamics is a powerful framework for describing the large-scale behaviours of many-body systems in inhomogeneous, non-stationary states. Until recently, however, it was restricted to non-integrable models, as the assumption of local thermodynamic equilibrium is broken by the large amount of conserved charges afforded by integrability. I will describe how to generalise hydrodynamics to integrable systems. The resulting theory has a rich structure, and applies to large families of quantum and classical field theories, chains and gases. It allows us to solve experimentally relevant setups such as the famous ``quantum Newton's cradle" in cold atomic gases, and to evaluate exact non-equilibrium currents, correlations, Drude weights and full counting statistics of fluctuations in non-equilibrium transport. If time permits, I will explain the latter, which is based on new very general developments showing how linear fluctuating hydrodynamics gives access to the exact large deviation theory of ballistic transport.
Nabil IqbalDurham UniversityGeneralized global symmetries, Goldstone modes, and photonization
Just as ordinary global symmetries are associated with a conserved particle number, quantum field theories with generalized global symmetries have a conserved density of higher-dimensional objects (such as strings, branes, etc.). I will discuss the emergence of gapless Goldstone modes when such a symmetry is spontaneously broken and will review how such a generalized symmetry plays an important role in characterizing the long-distance physics of familiar Maxwell electrodynamics in four dimensions. Many structures of ordinary symmetries admit a higher-form generalization; I will discuss some of these, focusing on the 4d analogues of familiar 2d concepts such as bosonization and (Abelian) Kac-Moody algebras. If time permits I will discuss the holographic realizations of such higher-form symmetries for field theories with gravity duals.
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ICMSLecture Theatre2019-01-23Preseminar for PhD studentsJosé Figueroa-O'Farrill University of EdinburghHomogeneous kinematical spacetimesWhat are the possible classical symmetries of space and time? I will discuss a partial answer to this question by explaining a classification of simply-connected homogeneous spatially isotropic spacetimes obtained recently in collaboration with Stefan Prohazka. The talk will focus on the main results and not so much on the details of how they were obtained. I will also comment on some BMS-like algebras of symmetries of some of these spacetimes. That is based on recent work with Stefan and Ross Grassie.
Benjamin McKeeverJohannes Gutenberg University MainzSimple theory of a magnetic skyrmion excitation
During the breathing-like behaviour of chiral skyrmions and antiskyrmions in ferromagnets the core of the spin structure periodically grows and shrinks in time. We present a Hamiltonian formulation for magnetization dynamics in collective coordinates and derive from it a single mode approximation for the breathing behaviour, where skyrmions and antiskyrmions obey the same dynamical equations. The analysis is supported by comparison to full micromagnetic simulations of the Landau-Lifshitz-Gilbert field equation. We identify two regimes with different kinds of breathing oscillations and derive the periods and amplitudes for both. The motion can be understood intuitively from the view of an effective energy landscape. In a high energy rotating regime the spin structure breathes along with a non-uniform precession of the skyrmion phase along with a linear energy dissipation on average. In comparison in the low energy oscillatory regime the skyrmion phase does not precess but oscillates around the stable equilibrium and we find an exponential energy decay associated with damped harmonic oscillations. This is loosely analogous to a damped pendulum that fully rotates around its suspension point until it eventually does not possess enough energy to fully rotate past the “upside-down” unstable fixed point and thereafter only oscillates back and forth.
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ICMSLecture Theatre2019-02-06
Preseminar for PhD students
Bernd Schroers Heriot-Watt UniversityA solvable model for magnetic skyrmions
Magnetic skyrmions are topological solitons which occur in a large class of ferromagnetic materials and which are currently attracting much attention in the condensed matter community. The talk is about an integrable model for magnetic skyrmions, introduced in a recent paper with Bruno Barton-Singer and Calum Ross. Solutions can be written in terms of a fixed anti-holomorphic and an arbitrary holomorphic function. I will explain the model and the geometry behind, and exhibit some of the solutions.
Shahar HadarHarvard University
Near-extremal black holes at late times, backreacted
Linear perturbations of extremal black holes exhibit the Aretakis instability, in which higher derivatives of the fields grow polynomially with time along the event horizon. Near-extremal black holes display similar behavior for some time, and eventually decay exponentially through quasinormal modes. In the talk I will show that the above behaviors are dictated by the conformal symmetry of the near-horizon region of such black holes. I will then discuss the significance of backreaction in the problem, and show how it can be simply accounted for within the near-horizon picture.
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ICMSSeminar Room2019-02-20Preseminar for PhD studentsJoan CampsUniversity College LondonSuperselection Sectors of Gravitational Subregions
Motivated by the problem of defining the entanglement entropy of the graviton, we study the division of the phase space of general relativity across subregions. Our key requirement is demanding that the separation into subregions is imaginary---i.e., that entangling surfaces are not physical. This translates into a certain condition on the symplectic form. We find that gravitational subregions that satisfy this condition are bounded by surfaces of extremal area. We characterise the 'centre variables' of the phase space of the graviton in such subsystems, which can be taken to be the conformal class of the induced metric in the boundary, subject to a constraint involving the traceless part of the extrinsic curvature. We argue that this condition works to discard local deformations of the boundary surface to infinitesimally nearby extremal surfaces, that are otherwise available for generic codimension-2 extremal surfaces of dimension ≥ 2.
Benoit VicedoUniversity of YorkIntegrable coupled sigma-models
I will begin by reviewing how classical integrable sigma models can be recast as classical Gaudin models associated with affine Kac-Moody algebras, or affine Gaudin models for short. One usually thinks of the Gaudin model as a spin chain, since it can be obtained from a certain limit of the XXX spin chain. When viewed in this way, integrable sigma models are essentially described by affine Gaudin models with a single site. It is then very natural, in this formalism, to consider affine Gaudin models with arbitrarily many sites. I will then go on to show that such multi-site classical Gaudin models can be used to construct new relativistic classical integrable field theories that couple together an arbitrary number of integrable sigma models. This talk is based on joint work, arXiv:1811.12316 and to appear, with F. Delduc, M. Magro and S. Lacroix. UE -->
ICMSLecture Theatre2019-03-06Preseminar for PhD studentsArjun BagchiIndian Institute of Technology KanpurSergey CherkisUniversity of ArizonaUE -->
ICMSLecture Theatre2019-03-20Preseminar for PhD studentsDaniele ValeriUniversity of GlasgowNiels ObersNiels Bohr InstituteUE -->
ICMSSeminar Room2019-04-03Preseminar for PhD studentsRichard DavisonCambridgeSebastian LautzKing's College LondonUE -->
ICMSLecture Theatre2019-04-17Preseminar for PhD studentsTamás Görbe University of LeedsAlec Barns-GrahamCambridgeUE -->
ICMSSeminar Room2019-05-01Preseminar for PhD studentsCohl FureyUniversity of CambridgeAlejandra CastroUniversity of AmsterdamUE -->
ICMSSeminar Room2019-05-15Preseminar for PhD studentsAndrew RoystonPenn State FayetteAllan GerrardUniversity of York