On the 11th of March UMONS will host the Belgian 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 Simone Speziale (CPT Marseille), Céline Zwikel (Collège de France) and Biswajit Sahoo (King’s College London). The seminars will take place in the Mirzakhani room of the building de Vinci in the morning and in the La Fontaine room of the Vésale building in the afternoon:

10h30 (Mirzakhani room, building 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 (Mirzakhani room, building 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 (La Fontaine room, building 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”.