Speaker
Description
A $t$-channel singularity of a cross section occurs in a $2\to 2$ process when the mediator is allowed to be on-shell, i.e. when the process can be treated as a sequence of a $1\to 2$ decay and a $2\to 1$ inverse decay. If, moreover, the mediator is stable, this singularity cannot be regularized within the common Breit-Wigner approach.
In this talk I will discuss the conditions for the singularity to occur and briefly summarize attempts (proposed in literature) to regularize it in case of collider physics and cosmological considerations of a thermal medium of particles. After showing that none of previously proposed ways to solve the problem is satisfactory in the cosmological case, I will present a natural solution developed within the Keldysch-Schwinger formalism: a non-zero imaginary part of the mediator's self-energy that appears as a consequence of interactions between the mediator and the thermal medium. Consequently, the mediator acquires a non-zero effective decay width and the cross section becomes finite.
