Thermal production of relativistic Majorana neutrinos: Strong enhancement by multiple soft scattering

TL;DR

This paper investigates the thermal production of relativistic Majorana neutrinos in the early Universe, revealing that multiple soft gauge interactions significantly enhance the production rate beyond traditional decay processes.

Contribution

It introduces a comprehensive calculation including multiple soft gauge interactions, showing their leading order contribution and resulting in a substantial rate enhancement.

Findings

Production rate is smooth where 1<->2 processes are forbidden.

Rate is enhanced by a factor of 3 at high temperatures.

Multiple soft interactions dominate the production mechanism.

Abstract

The production rate of heavy Majorana neutrinos is relevant for models of thermal leptogenesis in the early Universe. In the high temperature limit the production can proceed via the 1 <-> 2 (inverse) decays which are allowed by the thermal masses. We consider new production mechanisms which are obtained by including additional soft gauge interactions with the plasma. We show that an arbitrary number of such interactions gives leading order contributions, and we sum all of them. The rate turns out to be smooth in the region where the 1 <-> 2 processes are kinematically forbidden. At higher temperature it is enhanced by a factor 3 compared to the 1 <-> 2 rate.