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Leptogenesis and the Small-Angle MSW Solution

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.