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