Heavy Majorana Neutrinos and Baryogenesis ∗

The scenario of baryogenesis through leptogenesis is reviewed in models involving heavy Majorana neutrinos. The various mechanisms of CP violation occurring in the outof-equilibrium lepton-number-violating decays of heavy Majorana neutrinos are studied within a resummation approach to unstable-particle mixing. It is explicitly demonstrated how the resummation approach preserves crucial field-theoretic properties such as unitarity and CPT invariance. Predictions of representative scenarios are presented after solving numerically the Boltzmann equations describing the thermodynamic evolution of the Universe. The phenomenological consequences of heavy Majorana neutrinos on low-energy observables, such as lepton-flavour and/or lepton-number nonconservation in τ and Z-boson decays and electron electric dipole moment, are discussed.