Leptogenesis with virtual Majorana neutrinos - art. no. 016005

We present a mechanism of leptogenesis based on the out-of-equilibrium deca y of a scaler particle into heavy virtual Majorana neutrinos. This scheme p resents many conceptual advantages over the conventional scenario of Fukuei ta and Yanagida. In particular, the standard techniques of quantum field th eory can be used to compute the lepton asymmetry, without resorting to the phenomenological approximations usually made to describe unstable particles . This simplification allows us to address in a well-defined framework some issues raised in the recent literature. We also show, in a toy model, that a successful leptogenesis scenario is possible and requires a rather light scalar particle, 10(6) GeV<m<10(13) GeV. A natural embedding of this schem e in a gauged unified theory encompassing the Majorana fermions seems, howe ver, difficult. [S0556-2821(99)01913-X].