# Joint hep-th seminars with ULB, VUB, KUL and UGent

On the 31st of May UMONS will host the joint theoretical high-energy physics seminars organised by the Service de Physique de l’Univers, Champs et Gravitation of UMONS, the Service de Physique Théorique and the Service de Physique Théorique et Mathématique, both of the ULB, the Theoretical Particle Physics Group of the VUB, the High Energy Physics and Relativistic Field Theory group of the KUL, the Theoretical High Energy Physics group of the UGent and the International Solvay Institutes.

The speakers will be **Michele Schiavina** (University of Pavia) and **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.

7000 Mons, Belgium