ar X iv : h ep - p h / 05 01 22 1 v 1 2 4 Ja n 20 05 Radiative leptogenesis in minimal seesaw models ∗

Recently, a lot of attention has been payed to the study of a possible connection between thermal leptogenesis and neutrino masses [2], mixing and leptonic CP violation [3]. For instance, it is now known that an acceptable value for the baryon asymmetry of the Universe (BAU) requires the low-energy Majorana neutrinos to be lighter than 0.12 − 0.15 eV. Moreover, successful thermal leptogenesis implies M1 > 10 8 − 10 GeV [2,4], where M1 denotes the mass of the lightest heavy Majorana neutrino. Although interesting, these bounds are model-dependent in the sense that they are only valid if the heavy Majorana neutrino masses are hierarchical. In supersymmetric theories, the above constraint on M1 may be in conflict with the upper bound on the reheating temperature of the Universe, which can be as low as 10 GeV [5]. This tension between the bounds on M1 and TRH can be relaxed if one considers quasi-degenerate heavy Majorana neutrinos. In this case, acceptable values for the BAU can be obtained with heavy masses as low as 1 TeV [6]. In Ref. [1] we have studied the case where the small heavy Majorana neutrino mass-splitting, needed to enhance the CP -asymmetries, is generated radiatively. For simplicity, we have restricted ourselves to a seesaw model with only two