Updated BBN bounds on the cosmological lepton asymmetry for non-zero theta13

TL;DR

This paper revises bounds on the cosmological lepton asymmetry considering recent measurements of the neutrino mixing angle theta13, analyzing how it influences neutrino distributions, effective neutrino number, and primordial element yields.

Contribution

It provides updated bounds on lepton asymmetry by incorporating recent theta13 data and detailed flavor oscillation dynamics during Big Bang Nucleosynthesis.

Findings

Larger effective neutrino number N_eff possible for certain theta13 values.

Upper bounds on N_eff depend on neutrino mass hierarchy.

Detectable N_eff values by Planck only for very small theta13.

Abstract

We discuss the bounds on the cosmological lepton number from Big Bang Nucleosynthesis (BBN), in light of recent evidences for a large value of the neutrino mixing angle theta13. The largest asymmetries for electron and muon or tau neutrinos compatible with 4He and 2H primordial yields are computed versus the neutrino mass hierarchy and mixing angles. The flavour oscillation dynamics is traced till the beginning of BBN and neutrino distributions after decoupling are numerically computed. The latter contains in general, non thermal distortion due to the onset of flavour oscillations driven by solar squared mass difference in the temperature range where neutrino scatterings become inefficient to enforce thermodynamical equilibrium. Depending on the value of theta13, this translates into a larger value for the effective number of neutrinos, N_eff. Upper bounds on this parameter are discussed for both neutrino mass hierarchies. Values for N_eff which are large enough to be detectable by the Planck experiment are found only for the (presently disfavoured) range sin^2(theta13)<0.01.