Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
2001-08-22
Phys.Rev. D65 (2002) 043509; Addendum-ibid. D67 (2003) 127301
Physics
High Energy Physics
High Energy Physics - Phenomenology
29 pages, 3 figures; to appear on Phys.Rev.D; added discussion (at page 19) and references; typos corrected
Scientific paper
From the current cosmological observations of CMB and nuclear abundances we show, with an analytic procedure, that the total effective number of extra neutrino species $\Delta N_{\nu}^{\rm tot}< 0.3$. We also describe the possible signatures of non standard effects that could be revealed in future CMB observations. This cosmological information is then applied to neutrino mixing models. Taking into account the recent results from the SNO and SuperKamiokande experiments, disfavouring pure active to sterile neutrino oscillations, we show that all 4 neutrino mixing models, both of 2+2 and 3+1 type, lead to a full thermalization of the sterile neutrino flavor. Moreover such a sterile neutrino production excludes the possibility of an electron neutrino asymmetry generation and we conclude that $\Delta N_{\nu}^{\rm tot}\simeq 1$, in disagreement with the cosmological bound. This result is valid under the assumption that the initial neutrino asymmetries are small. We suggest the existence of a second sterile neutrino flavor, with mixing properties such to generate a large electron neutrino asymmetry, as a possible way out.
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