Neutrino Mixing and Cosmology
نویسنده
چکیده
The relic neutrino background has never been directly detected, so we must resort to indirect means to infer its properties. One of the most useful tools available is Big Bang Nucleosynthesis (BBN). By putting neutrino oscillations together with BBN, we may shed light on neutrino mixing, cosmology, or both. We will outline the consequences of oscillations of the relic background neutrinos, for both active-active and active-sterile oscillation modes. The central issue we shall probe is: how big is the universe’s lepton number? While the baryon asymmetry of the universe is well determined, nB/nγ ≃ 5 × 10 , the size of the lepton asymmetry is unknown. The simplest assumption is that the baryon and lepton asymmetries are of the same size, as would be the case ifB−L were conserved. However, there are viable models in which L may be large while B is small. Given constraints on charge neutrality, any large lepton asymmetry would have to be hidden in the neutrino sector. Since neutrinos and antineutrinos should be in chemical equilibrium until they decouple at a temperature T ∼ 2 MeV, they may be welldescribed by Fermi-Dirac distributions with equal and opposite chemical potentials:
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