Thermal production of ultrarelativistic right-handed neutrinos: Complete leading-order results

TL;DR

This paper completes the leading-order calculation of relativistic right-handed neutrino production in the early Universe by including 2 -> 2 scattering processes, highlighting the dominant role of gauge interactions.

Contribution

It extends previous work by incorporating 2 -> 2 scattering processes into the leading-order production rate calculation for right-handed neutrinos.

Findings

Gauge interactions dominate the production rate.

Soft gauge boson exchange significantly enhances production.

A new sum rule for the HTL resummed fermion propagator is derived.

Abstract

The thermal production of relativistic right-handed Majorana neutrinos is of importance for models of thermal leptogenesis in the early Universe. Right-handed neutrinos can be produced both by 1 <-> 2 decay or inverse decay and by 2 -> 2 scattering processes. In a previous publication, we have studied the production via 1 <-> 2 (inverse) decay processes. There we have shown that multiple scattering mediated by soft gauge boson exchange also contributes to the production rate at leading order, and gives a strong enhancement. Here we complete the leading order calculation by adding 2 -> 2 scattering processes involving either electroweak gauge bosons or third-generation quarks. We find that processes with gauge interactions give the most important contributions. We also obtain a new sum rule for the Hard Thermal Loop resummed fermion propagator.