Joint hep-th seminars with ULB, VUB and KUL

On the 30th of November 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 and the International Solvay Institutes.

The speakers will be Oliver Schlotterer (Uppsala University) and Manus Visser (University of Cambridge):

10h30: Oliver Schlotterer

Modular graph forms and iterated integrals in string amplitudes

I will discuss string amplitudes as a laboratory for special functions and periods integrals that drive fruitful cross-talk with particle physicists and mathematicians. At genus zero, integration over punctures on a disk or sphere worldsheet generates multiple zeta values in the low-energy expansion of open- and closed-string amplitudes. At genus one, closed-string amplitudes introduce infinite families of non-holomorphic modular forms through the integration over torus puctures known as modular graph forms. The latter inspired Francis Brown’s alternative construction of non-holomorphic modular forms in the mathematics literature, and I will report on recent progress in clarifying their connection with modular graph forms.

12h00: Manus Visser

Thermodynamic ensembles for de Sitter space and other causal diamonds

In 1977 Gibbons and Hawking (GH) famously derived the entropy of black hole and de Sitter (dS) horizons from a gravitational partition function. While their result is clearly correct, in the case of the dS horizons, the statistical foundation and interpretation of their approach has remained obscure, since there is no boundary in (Euclidean) dS at which to define the temperature or energy of the ensemble. Following recent work by Jacobson and Banihashemi I will explain how this issue can be addressed by introducing an artificial timelike boundary at which a thermodynamic ensemble can be defined, as has been done previously by York for black hole ensembles. In the limit where the boundary vanishes this provides a proper thermodynamic interpretation of the GH partition function, and I will argue it resolves a confusion about the minus sign in the first law of dS horizons. Moreover, I will show how the GH approach can be generalized to dS black holes and causal diamonds using the formalism of constrained instantons.