Quantum effects in the generation of the matter-antimatter asymmetry

According to leptogenesis scenario, the observed excess of matter over antimatter has been produced by the decay of heavy Majorana neutrinos. If masses of the heavy neutrinos are quasi-degenerate then the self-energy contribution to the CP-violating parameter is resonantly enhanced and it is possible to have successful leptogenesis at a relatively low scale. In the canonical approach the CP-violating parameter is calculated in vacuum, which is a questionable approximation for processes in hot and dense early Universe. Using methods of non-equilibrium quantum field theory we find that the canonical expression for the CP-violating parameter is only applicable in the hierarchical case even though it does not diverge in the limit of degenerate masses. In the resonant regime one has to take into account medium corrections to the masses and decay widths, which yields a modified expression for the CP-violating parameter. Another important effect is the resonant and medium enhancement of the total decay widths. It leads to a faster decay of the heavy particles and increased washout of the generated asymmetry. Therefore, the amplified asymmetry production due to the enhancement of the CP-violating parameter is partially compensated by the increase of the in-medium decay widths.