Baryogenesis by heavy-neutrino decay and sphaleron reprocessing of both
baryon and lepton number is reconsidered, paying special attention to the
flavour structure of the general evolution equations and developing an
approximate but sufficiently accurate analytic solution to the prototype
evolution equation. Two different models of neutrino masses are examined, based
on an Abelian U(1) or a non-Abelian U(2) family symmetry. We show that a
consistent picture of baryogenesis can emerge in both cases, although with
significant differences.Comment: 14 pages. v4: revised using the corrected Boltzmann equations of
hep-ph/031012
