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 M-1 of the decaying heavy Majorana neutrino, the effective light neutrino mass (m) over tilde (1) and the absolute mass scale (m) over bar of the light neutrinos. In the mass range suggested by neutrino oscillations, msol similar or equal to 8 x 10(-3) eV less than or similar to (m) over tilde (1) less than or similar to m(atm) less than or similar to 5 x 10(-2) eV, 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 M-1. We also derive an analytical expression for the upper bound on the light neutrino masses implied by successful leptogenesis. (C) 2004 Elsevier Inc. All rights reserved.