Kadanoff-Baym approach to the thermal resonant leptogenesis
  Using the non-equilibrium Green function method (Kadanoff-Baym equations) in
the expanding universe, we investigate evolution of the lepton number asymmetry
when the right-handed (RH) neutrinos have almost degenerate masses
$|M_i^2-M_j^2| \ll M_i^2$. The resonantly enhanced $CP$-violating parameter
$\varepsilon_i$ associated with the decay of the RH neutrino $N_i$ is obtained.
It is proportional to an enhancement factor $(M_i^2-M_j^2) M_i \Gamma_j/
((M_i^2-M_j^2)^2 +R_{ij}^2)$ with the regulator $R_{ij}=M_i \Gamma_i+M_j
\Gamma_j$. The result is consistent with the previous result obtained by Garny
et al., in a constant background with an out-of-equilibrium initial state. We
discuss the origin of such a regulator, and why it is not like $R_{ij}=M_i
\Gamma_i-M_j \Gamma_j$.
