The B-L Phase Transition: Implications for Cosmology and Neutrinos
  We investigate the possibility that the hot thermal phase of the early
universe is ignited in consequence of the B-L phase transition, which
represents the cosmological realization of the spontaneous breaking of the
Abelian gauge symmetry associated with B-L, the difference between baryon
number B and lepton number L. Prior to the B-L phase transition, the universe
experiences a stage of hybrid inflation. Towards the end of inflation, the
false vacuum of unbroken B-L decays, which entails tachyonic preheating as well
as the production of cosmic strings. The dynamics of the B-L breaking Higgs
field and the B-L gauge degrees of freedom, in combination with thermal
processes, generate an abundance of heavy (s)neutrinos. These (s)neutrinos
decay into radiation, thereby reheating the universe, generating the baryon
asymmetry of the universe and setting the stage for the thermal production of
gravitinos. We study the B-L phase transition in the full supersymmetric
Abelian Higgs model, for which we derive and discuss the Lagrangian in
arbitrary and unitary gauge. As for the subsequent reheating process, we
formulate the complete set of Boltzmann equations, the solutions of which
enable us to give a detailed and time-resolved description of reheating.
Assuming the gravitino to be the lightest superparticle (LSP), the requirement
of consistency between hybrid inflation, leptogenesis and gravitino dark matter
implies relations between neutrino and superparticle masses, in particular a
lower bound on the gravitino mass of 10 GeV. Similarly, in the case of very
heavy gravitinos, the nonthermal production of pure wino or higgsino LSPs in
heavy gravitino decays can account for the observed amount of dark matter,
while simultaneously fulfilling the constraints imposed by primordial
nucleosynthesis and leptogenesis.
