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

Abstract

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