Flavour mixing transport theory and resonant leptogenesis
  We derive non-equilibrium quantum transport equations for flavour-mixing
fermions. We develop the formalism mostly in the context of resonant
leptogenesis with two mixing Majorana fermions and one lepton flavour, but our
master equations are valid more generally in homogeneous and isotropic systems.
We give a hierarchy of quantum kinetic equations, valid at different
approximations, that can accommodate helicity and arbitrary mass differences.
In the mass-degenerate limit the equations take the familiar form of density
matrix equations. We also derive the semiclassical Boltzmann limit of our
equations, including the CP-violating source, whose regulator corresponds to
the flavour coherence damping rate. Boltzmann equations are accurate and
insensitive to the particular form of the regulator in the weakly resonant case
$\Delta m \gg \Gamma$, but for $\Delta m \lesssim \Gamma$ they are
qualitatively correct at best, and their accuracy crucially depends on the form
of the CP-violating source.
