Leptogenesis and the Small-Angle MSW Solution

TL;DR

This paper explores how lepton asymmetry from heavy Majorana neutrino decay can produce the universe's baryon asymmetry, focusing on specific neutrino mixing and mass assumptions, and deriving bounds on neutrino masses for successful baryogenesis.

Contribution

It provides new bounds on heavy Majorana neutrino masses necessary for baryogenesis under the small-angle MSW solution and hierarchical neutrino masses.

Findings

Successful baryogenesis requires M_2, M_3 > 10^{11} GeV with symmetric Dirac matrices.

If any Dirac matrix element vanishes, baryogenesis can occur with M_2, M_3 as low as a few times 10^{9} GeV.

Results are compatible with supersymmetric cosmology reheat constraints.

Abstract

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.