Leptogenesis for pedestrians

During the process of thermal leptogenesis temperature decreases by about one order of magnitude while the baryon asymmetry is generated. We present an analytical description of this process so that the dependence on the neutrino mass parameters becomes transparent. In the case of maximal CP asymmetry all decay and scattering rates in the plasma are determined by the mass M1 of the decaying heavy Majorana neutrino, the effective light neutrino mass m∼1 and the absolute mass scale m¯ of the light neutrinos. In the mass range suggested by neutrino oscillations, msol≃8×10-3eV≲m∼1≲matm≃5×10-2eV, leptogenesis is dominated just by decays and inverse decays. The effect of all other scattering processes lies within the theoretical uncertainty of present calculations. The final baryon asymmetry is dominantly produced at a temperature TB which can be about one order of magnitude below the heavy neutrino mass M1. We also derive an analytical expression for the upper bound on the light neutrino masses implied by successful leptogenesis.