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