The Thermal Abundance of Semi-Relativistic Relics
  Approximate analytical solutions of the Boltzmann equation for particles that
are either extremely relativistic or non-relativistic when they decouple from
the thermal bath are well established. However, no analytical formula for the
relic density of particles that are semi-relativistic at decoupling is yet
known. We propose a new ansatz for the thermal average of the annihilation
cross sections for such particles, and find a semi-analytical treatment for
calculating their relic densities. As examples, we consider Majorana- and
Dirac-type neutrinos. We show that such semi-relativistic relics cannot be good
cold Dark Matter candidates. However, late decays of meta-stable
semi-relativistic relics might have released a large amount of entropy, thereby
diluting the density of other, unwanted relics.
