Four-neutrino mixing and Big-Bang Nucleosynthesis

Abstract

We investigate the Big-Bang Nucleosynthesis constraints on neutrino mixing in the framework of the two four-neutrino schemes that are favored by the results of neutrino oscillation experiments. We discuss the implications of these constraints for terrestrial short and long-baseline neutrino oscillation experiments and we present some possibilities of testing them in these experiments. In particular, we show that from the Big-Bang Nucleosynthesis constraints it follows that the (−) ν µ → (−) ν τ transition is severely suppressed in short-baseline experiments, whereas its oscillation amplitude in long-baseline experiments is of order 1. We also propose a new parameterization of the four-neutrino mixing matrix U which is appropriate for the schemes under consideration.

Four-neutrino oscillation solutions of the atmospheric neutrino anomaly

“…Such sign can instead be relevant in other contexts, such as in supernovae (see, e.g., [21]) in big-bang nucleosynthesis (BBN) (see, e.g., [22,23] and refs. therein) and, for Majorana neutrinos, in neutrinoless double beta decay (0ν2β) (see, e.g., [24][25][26] and refs.…”

Neutrino masses and mixing from the astrophysical perspective

“…Such a state can be thermally excited by the usual interplay of oscillations and collisions [35]. The range of mass differences and mixing angles where a significant population of r.h. neutrinos would be excited has been studied, e.g., by Enqvist, Kainulainen and Thomson [36] and more recently by Bilenky et al [37]. For example, a v,-v,-oscillation solution of the atmospheric neutrino anomaly would have led to a possible conflict with BBN, but this solution is now anyway experimentally disfavored.…”

Neutrinoless double-β decay with three or four neutrino mixing

“…with four light and mixed neutrinos [52][53][54][55][56][57][58], which constitute the minimal possibility that allows to explain all the data of neutrino oscillation experiments. In this case, in the flavor basis the three active neutrinos ν e , ν µ , ν τ are accompanied by a sterile neutrino ν s that does not take part in standard weak interactions.…”

“…Furthermore, the upper bound N BBN ν < 4 for the effective number of neutrinos in BigBang Nucleosynthesis (BBN) (see, for example, [60]) implies that [54,57,58] 19) in scheme A. The analysis of recent astrophysical data yields the upper bound 3 N BBN ν ≤ 3.2 at 95% CL [61], although the issue is still rather controversial (see [62,63]).…”