Solar neutrino spectrum, sterile neutrinos and additional radiation in the Universe

TL;DR

This paper explores how very light sterile neutrinos could explain the lack of expected spectral upturns in solar neutrino data and account for additional relativistic degrees of freedom suggested by cosmological observations.

Contribution

It proposes a specific sterile neutrino mixing model that explains solar neutrino spectral features and cosmological data, and discusses testable predictions for future experiments.

Findings

Sterile neutrino mixing can suppress the spectral upturn in solar neutrino data.

The model accounts for additional relativistic degrees of freedom in the early Universe.

Predictions for future neutrino experiments can test the sterile neutrino hypothesis.

Abstract

Recent results from the SNO, Super-Kamiokande and Borexino experiments do not show the expected upturn of the energy spectrum of events (the ratio $R \equiv N_{obs}/N_{SSM}$) at low energies. At the same time, cosmological observations testify for possible existence of additional relativistic degrees of freedom in the early Universe: $\Delta N_{eff} = 1 - 2$. These facts strengthen the case of very light sterile neutrino, $\nu_s$, with $\Delta m^2_{01} \sim (0.7 - 2) \cdot 10^{-5}$ eV$^2$, which mixes weakly with the active neutrinos. The $\nu_s$ mixing in the mass eigenstate $\nu_1$ characterized by $\sin^2 2\alpha \sim 10^{-3}$ can explain an absence of the upturn. The mixing of $\nu_s$ in the eigenstate $\nu_3$ with $\sin^2 \beta \sim 0.1$ leads to production of $\nu_s$ via oscillations in the Universe and to additional contribution $\Delta N_{eff} \approx 0.7 - 1$ before the big bang nucleosynthesis and later. Such a mixing can be tested in forthcoming experiments with the atmospheric neutrinos as well as in future accelerator long baseline experiments. It has substantial impact on conversion of the supernova neutrinos.