The lepton asymmetry created in the out-of-equilibrium decay of a heavy
Majorana neutrino can generate the cosmological baryon asymmetry when processed
through fast anomalous electroweak reactions. In this work I examine this
process under the following assumptions: (1) maximal nu_mu/nu_tau mixing (2)
hierarchical mass spectrum m_3 >> m_2 (3) small-angle MSW solution to the solar
neutrino deficit. Working in a basis where the charged lepton and heavy
neutrino mass matrices are diagonal, I find the following bounds on the heavy
Majorana masses M_i: (a) for a symmetric Dirac neutrino mass matrix (no other
constraints), an asymmetry compatible with BBN constraints can be obtained for
min(M_2,M_3)> 10^{11} GeV; (b) if {\em any} of the Dirac matrix elements
vanishes, successful baryogenesis can be effected for a choice of min(M_2,M_3)
as low as a few times 10^{9} GeV. The latter is compatible with reheat
requirements for supersymmetric cosmologies with sub-TeV gravitino masses.Comment: 12 pages, LaTeX; version to be published in Physics Letters
