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

Le 11 mars, 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 et les Instituts Internationaux Solvay.

Les orateurs seront Simone Speziale (CPT Marseille), Céline Zwikel (Collège de France) et Biswajit Sahoo (King’s College London). Les séminaires auront lieu le matin dans la salle Mirzakhani du bâtiment de Vinci et l’après-midi dans la salle La Fontaine du bâtiment Vésale.

10h30 (salle Mirzakhani, bâtiment de Vinci): Céline Zwikel

A covariant formulation of logarithmic supertranslations at spatial infinity

In this talk, I will investigate the asymptotic symmetries of four-dimensional asymptotically flat spacetimes at spatial infinity using covariant phase space methods. I will introduce a new symplectic structure together with consistent boundary conditions that realize novel symmetries—beyond those identified at null infinity—namely logarithmic translations and log-supertranslations. The associated charges are finite and conserved, and we show that their algebra admits non-trivial central extensions.

12h00 (salle Mirzakhani, bâtiment de Vinci): Simone Speziale

The importance of covariance for gravitational charges

When prescribing charges for gravitational symmetries, it is very important to include a criterium of covariance. This ensures background-independence, and is often strong enough to remove all ambiguities. I will show examples of how a covariant prescription removes field-dependent cocycles, resolves the O(G^3) controversy in the post-Minkowskian expansion, and selects a corner improvement of the symplectic 2-form for extended BMS.

14h30 (salle La Fontaine, bâtiment Vésale): Biswajit Sahoo

Emergent Higher-Spin Symmetry on the Celestial Sphere from Loop-Corrected Soft Photon Theorems

In this talk, I will briefly outline the derivation of logarithmic soft photon theorems by analyzing loop amplitudes of four-dimensional QED S-matrices describing the scattering of massive charged particles. I will then explain how these logarithmic soft factors are directly related to the early- and late-time components of electromagnetic waveforms in classical scattering processes, leading to a new prediction for electromagnetic tail memory effect. In the second part of the talk, I will discuss how the all-loop logarithmic soft photon theorems, particularly in the high-energy scattering limit, admit a natural interpretation as Ward identities in the associated « celestial CFT ». Requiring the locality of these Ward identities leads to the emergence of an infinite tower of higher-spin conserved currents on the celestial sphere. I will show that these currents generate a (w_{1+\infty}) algebra, revealing an emergent higher-spin symmetry structure that arises purely from quantum (loop) effects. This provides a concrete example of how infrared dynamics and loop-level soft theorems organize themselves into an infinite-dimensional symmetry algebra within the framework of « celestial holography ».