NEUTRINO ASYMMETRY GENERATION IN THE EARLY UNIVERSE (FROM να ↔ νs OSCILLATIONS)

Neutrino asymmetry generation from active-sterile neutrino oscillations in the early Universe has different important cosmological effects. We discuss its basic features and present recent results on the borders of the generation for mixing angles sin 2 2θ0 ≤ 10 −7. 1 General conditions for neutrino asymmetry generation Let us consider, from a statistical point of view, a two neutrino mixing ν α ↔ ν β in the early Universe as a special elementary process violating flavor lepton number conservation but still preserving the total lepton number conservation in a way that L να + L ν β = const. Since the amplitude of the direct processes is equal to that one of the inverse processes, one expects that at the equilibrium the number of ν α is equal to that of ν β and the same for anti-neutrinos. This clearly implies also the equality of the asymmetries a in a way that L eq να = L eq ν β = const/2. There is however the possibility that neutrinos and anti-neutrinos mixings are different. In this case the two different equilibrium conditions for particle numbers can be reached at different times for neutrinos and anti-neutrinos. In the case that α and β are two ordinary neutrino flavors, they are both in thermal equilibrium conditions. This means that if initially L να = L ν β = 0, then also particle numbers are necessarily equal. This is why an asymmetry cannot be generated from active-active neutrino oscillations. This is not true if one of the two flavors is a sterile neutrino flavor ν s whose number density is negligible compared to that of active neutrinos at temperature below the quark-hadron phase transition at around 150 MeV. In this way if active neutrinos and active antineutrinos are converted into sterile neutrinos and anti-neutrinos at different rates, the equilibrium for the particle numbers can be reached at different times for neutrinos and anti-neutrinos. This implies that during this relaxation phase the neutrino asymmetries can change.