Seminarium "Teoria cząstek elementarnych i kosmologia"

A t-channel singularity of a scattering process can occur when a stable mediator is kinematically allowed to be on-shell. Then, the cross section appears to be infinite which, obviously, cannot be a physical result. Since the mediator has no width, it is impossible to regularize the singularity using the Breit-Wigner approach.

In this talk, I will discuss the conditions that are necessary for the singularity to occur. I will briefly summarize attempts to regularize the singularity that are already present in literature, especially in the case of collider physics.

After showing that none of previously proposed ways to solve the problem is satisfactory in the cosmological case of a thermal medium of particles, I will present a natural solution developed within the Keldysch-Schwinger formalism. A non-zero imaginary part of the mediator's self-energy 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.

slides: https://mycloud.fuw.edu.pl/index.php/s/ZCarLbB3HiZHwj7