Séminaires conjoints de physique théorique des hautes énergies avec ULB, VUB, KUL and UGent

Le 31 mai, l’UMONS accueillera les séminaires conjoints de physique théorique des hautes énergies organisés par le Service de Physique de l’Univers, Champs et Gravitation de l’UMONS, le Service de Physique Théorique et le Service de Physique Théorique et Mathématique, tous deux de l’ULB, le Groupe de Physique Théorique des Particules de la VUB, le groupe de Physique des Hautes Energies et de Théories des Champs Relativistes de la KUL, le groupe de Physique Théorique des Hautes Energies de l’UGent et les Instituts Internationaux Solvay.

Les orateurs seront Michele Schiavina (University of Pavia) et Sašo Grozdanov (University of Edinburgh and University of Ljubljana):

10h30: Michele Schiavina

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.

12h00: Sašo Grozdanov

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.