  We present a mechanism of leptogenesis based on the out-of-equilibrium decay
of a scalar particle into heavy virtual Majorana neutrinos. This scheme
presents many conceptual advantages over the conventional scenario of Fukugita
and Yanagida. In particular, the standard techniques of quantum field theory
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 scheme in a
gauged unified theory encompassing the Majorana fermions seems however
difficult.
