This week

Tuesday

From Yang-Mills and Maxwell in de Sitter space electromagnetic knots

Regular Seminar Olaf Lechtenfeld (Hannover)

at:
15:00 City U.
room AG21
abstract:

I will review analytic SU(2) Yang-Mills solutions with finite action on de Sitter space from a new perspective. As a byproduct, all abelian solutions are classified and related with rational electromagnetic knots. In the Yang-Mills case, the gravitational backreaction is easily taken in to account as well.

New routes to scattering on curved backgrounds

Regular Seminar Tim Adamo (Imperial College)

at:
13:30 IC
room H503
abstract:

There are many reasons to consider perturbative QFT around curved backgrounds, but it is often difficult to perform explicit computations in these settings. Progress in the study of scattering amplitudes (around a trivial background) suggests alternative perspectives to space-time Lagrangians and Feynman rules which could enable progress in the study of scattering on curved backgrounds. I will discuss one such alternative, known as double copy, with a particular focus on gluon and graviton scattering around non-linear plane wave backgrounds

Wednesday

Witten, Cardy, and the Holonomy Saddle

Polygon Seminar Piljin Yi (KIAS)

at:
15:00 KCL
room S-1.27
abstract:

This talk will explore topological invariants of susy gauge theories, with some emphasis on index-like quantities and the notion of holonomy saddles. We start with 1d refined Witten index computations where the twisted partition functions typically show rational, rather than integral, behavior. We will explain how this oddity is a blessing in disguise and propose a universal tool for extracting the truely enumerative Witten indices. In part, this finally put to the rest a two-decade-old bound state problems which had originated from the M-theory hypothesis. Along the way, we resolve an old and critical conflict between Kac+Smilga and Staudacher/Pestun, circa 1999~2002, whereby the notion of holonomy saddles emerges and plays a crucial role. More importantly, the holonomy saddle prove to be universal features of supersymmetric gauge theories when the spacetime include a small circle. We explore them further for d=4, N=1 theories, with much ramifications on recent claims on Cardy exponents of their partition functions.

Thursday

L_infinity algebras, the BV formalism, and Classical Fields

Exceptional Seminar Martin Wolf (Surrey)

at:
11:00 KCL
room Bush House S 2.01
abstract:

I will first review the Batalin-Vilkovisky formalism and its mathematical foundations with an emphasis on higher algebraic structures and classical field theories. I will then move on and discuss recent developments in formulating higher gauge theory with Lie quasi-groupoids as gauge structure. Finally, I will explain how all these ideas can be combined with those of twistor theory to formulate maximally superconformal gauge theories in four and six dimensions by means of quasi-isomorphisms.

Twisting and localization in supergravity: equivariant cohomology of BPS black holes

Exceptional Seminar Imtak Jeon (KCL)

at:
11:50 KCL
room Bush House S 2.01
abstract:

I will talk about the formalism of supersymmetric localization in supergravity using the deformed BRST algebra defined in the presence of a supersymmetric background. The gravitational functional integral localizes onto the cohomology of a global supercharge Qeq, obeying (Qeq)2=H, where H is a global symmetry of the background. This construction naturally produces a twisted version of supergravity whenever supersymmetry can be realized off-shell. I will present the details of the twisted graviton multiplet and ghost fields for the superconformal formulation of four-dimensional N=2 supergravity. As an application of our formalism, we systematize the computation of the exact quantum entropy of supersymmetric black holes. In particular, we compute the one-loop determinant of the Qeq deformation operator for the off-shell fluctuations of the Weyl multiplet around the AdS2×S2 saddle.

3d Abelian Gauge theories at the Boundary

Exceptional Seminar Edoardo Lauria (Durham)

at:
14:00 KCL
room Bush House S 2.01
abstract:

A four-dimensional abelian gauge theory can be coupled to a 3d CFT with a U(1) symmetry living on a boundary. This coupling gives rise to a continuous family of boundary conformal field theories (BCFTs) parametrized by the gauge coupling τ and by the choice of the CFT in the decoupling limit. Upon performing an Electric-Magnetic duality in the bulk and going to the decoupling limit in the new frame, one finds a different 3d CFT on the boundary, related to the original one by Witten's SL(2, Z) action. In particular the cusps on the real τ axis correspond to the 3d gauging of the original CFT. We study general properties of this family of BCFTs. We show how to express bulk one and two-point functions, and the hemisphere free-energy, in terms of the two-point functions of the boundary electric and magnetic currents. Finally, upon assuming particle-vortex duality (and its fermionic version), we show how to turn this machinery into a powerful computational tool to study 3d gauge theories.

Entanglement Content of Quantum Particle Excitations

Exceptional Seminar Cecilia De Fazio (City)

at:
14:50 KCL
room Bush House S 2.01
abstract:

In the first part of my talk I will give a brief introduction to the main features of the Entaglement Entropy of a bipartite system in Integrable Quantum Field Theories and Conformal Field Theories. Secondly I will discuss the results my collaborators and I obtained in our two papers. In particular, I will consider the Entaglement Entropy of a single connected region of a finite bipartite system in excited states described by one-dimensional massive free theories with finite numbers of particles. I will show that in the limit of large volume and large length of the region the excess of entanglement due to the presence of the particles with respect to the ground state takes a simple form and admits a "q-bit interpretation".

ntegrability-protected adiabatic reversibility in quantum spin chains

Exceptional Seminar Andrea De Luca (Oxford)

at:
16:00 KCL
room Bush House S 2.01
abstract:

I consider the out-of-equilibrium dynamics of the Heisenberg anisotropic quantum spin chain threaded by a time-dependent magnetic flux. I will exploit the integrability of the model for any flux values to derive an exact description of the dynamics in the limit of slowly varying flux: the state of the system is described at any time by a time-dependent generalized Gibbs ensemble. Two dynamical regimes emerge according to the value of the anisotropy Δ. For |Δ|>1, reversibility is preserved: the initial state is always recovered whenever the flux is brought back to its initial value. On the contrary, for |Δ|<1, instabilities of quasiparticles produce irreversible dynamics as confirmed by the dramatic growth of entanglement entropy. In this regime, the standard GHD description becomes incomplete and we complement it via a maximum entropy production principle.

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Regular Seminar Marco Meineri (EPFL, Lausanne)

at:
14:00 QMW
room G O Jones 610
abstract:

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Friday

Hecke relations and Galois symmetries in Rational Conformal Field Theories and Modular Tensor Categories

Exceptional Seminar Jeff Harvey (The University of Chicago)

at:
11:30 QMW
room 610
abstract:

I will discuss some old and new relations between the characters, modular data and braiding and fusing matrices of rational conformal field theories and their associated modular tensor categories. These relations involve concepts from number theory which I will explain in the talk.