Low-Scale Leptogenesis and the Domain Wall Problem in Models with
  Discrete Flavor Symmetries
  We propose a new mechanism for leptogenesis, which is naturally realized in
some models with a flavor symmetry based on the discrete group A_4, where the
symmetry breaking parameter also controls the Majorana masses for the heavy
right handed (RH) neutrinos. During the early universe, for T>TeV, part of the
symmetry is restored, due to finite temperature contributions, and the RH
neutrinos remain massless and can be produced in thermal equilibrium even at
temperatures well below the most conservative gravitino bounds. Below this
temperature the phase transition occurs and they become massive, decaying out
of equilibrium and producing the necessary lepton asymmetry. Unless the
symmetry is broken explicitly by Planck-suppressed terms, the domain walls
generated by the symmetry breaking survive till the quark-hadron phase
transition, where they disappear due to a small energy splitting between
different vacua caused by the QCD anomaly.
