Coherent flavour oscillation and CP violating parameter in thermal resonant leptogenesis

Abstract

Solving the Kadanoff-Baym (KB) equations in a different method from our previous analysis, we obtain the CP violating parameter ε in the thermal resonant leptogenesis without assuming smallness of the off-diagonal Yukawa couplings. For that purpose, we first derive a kinetic equation for density matrix of RH neutrinos with almost degenerate masses M i (i = 1, 2) ~ M. If the deviation from thermal equilibrium is small, the differential equation is reduced to a linear algebraic equation and the density matrix can be solved explicitly in terms of the time variation of (local) equilibrium distribution function. The obtained (CP-violating parameter ε i is proportional to an enhancement factor (M i2 ∈-∈M j2)M i Γ j /((M i2 ∈-∈M j2) 2∈+∈R ij2) with a regulator R ij = M(Γ i + Γ j ), consistent with the previous analysis. The decay width can be determined systematically by the 1PI self-energy of the RH neutrinos in the 2PI formalism. © 2014 The Author(s).