## Addendum to: Update on neutrino mixing in the early Universe

### Abstract

In the light of the recent WMAP results we update the constraints on a class of non standard BBN models with a simultaneous combination of non standard neutrino distributions and extra effective number of neutrinos in the expansion rate. These models can be described in terms of the two parameters ∆Ntot ν, constrained by the primordial Helium abundance Yp measurement, and ∆N ρ ν, constrained by a combination of CMB and primordial Deuterium data. Small deviations from standard Big Bang Nucleosynthesis are suggested. Different non standard scenarios can be distinguished by a measurement of the difference ∆N fν ν = ∆Ntot ν − ∆Nρ ν. From the current data we estimate ∆N fν ν ≃ −1.4 +0.9 −1.4, mildly disfavouring solutions with a low expansion rate, characterized by ∆N fν ν = 0 and negative ∆N ρ ν. Activesterile neutrino mixing could be a viable explanation only for high values of Yp � 0.24. The existence of large positive neutrino chemical potentials ξi ∼ 0.05, implying ∆N ρ ν ≃ 0, would be a possible explanation of the data within the analyzed class of non standard 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. 1 I

Year: 2003
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