Update on neutrino mixing in the early universe (vol 65, art no 043509, 2002)

In the light of the recent WMAP results we update the constraints on a class of nonstandard big bang nucleosynthesis (BBN) models with a simultaneous combination of nonstandard neutrino distributions and an extra effective number of neutrinos in the expansion rate. These models can be described in terms of the two parameters DeltaN(nu)(tot), constrained by the primordial helium abundance Y-p measurement, and DeltaN(nu)(rho), constrained by a combination of cosmic microwave background and primordial deuterium data. Small deviations from standard big bang nucleosynthesis are suggested. Different nonstandard scenarios can be distinguished by a measurement of the difference DeltaN(nu)(nu)(f)=DeltaN(nu)(tot)-DeltaN(nu)(rho). From the current data we estimate DeltaN(nu)(nu)(f)similar or equal to-1.4(-1.4)(+0.9), mildly disfavoring solutions with a low expansion rate, characterized by DeltaN(nu)(nu)(f)=0 and negative DeltaN(nu)(rho). Active-sterile neutrino mixing could be a viable explanation only for high values of Y(p)greater than or similar to0.24. The existence of large positive neutrino chemical potentials xi(i)similar to0.05, implying DeltaN(nu)(rho)similar or equal to0, would be a possible explanation of the data within the analyzed class of nonstandard BBN models. Interestingly, it would also provide a way to evade the cosmological bounds for "class A 3+1" four neutrino mixing models. A scenario with a decaying sterile neutrino is also considered.