CP violation effects on the neutrino degeneracy parameters in the Early Universe

We explore possible CP violating effects, coming from the Dirac phase of the Maki–Nakagawa–Sakata–Pontecorvo matrix, on the neutrino degeneracy parameters, at the epoch of Big-Bang nucleosynthesis. We first demonstrate the conditions under which such effects can arise. In particular it requires that the initial muon and tau neutrino degeneracy parameters ξνξν differ. Then we solve numerically the kinetic equations for the three flavor neutrino density matrix with the goal of quantifying the impact of the Dirac phase on the electron neutrino degeneracy parameter ξνeξνe. The calculations include the vacuum term, the coupling to matter, the νν   interaction and the collisions. Effects on ξνeξνe up to almost 1% and on YpYp of about 0.1% are found, depending on the initial conditions.