Numerical estimate of minimal active-sterile neutrino mixing for sterile neutrinos at GeV scale

TL;DR

This paper estimates the minimal active-sterile neutrino mixing angles at GeV scale, considering CP-violating phases, to guide experimental sensitivity for sterile neutrino detection in fixed-target experiments.

Contribution

It extends previous analyses by including CP-violating phases and provides numerical estimates of minimal mixing angles, revealing a strong dependence on neutrino mass hierarchy and lightest neutrino mass.

Findings

Minimal mixing angles can be as low as 10^{-20} in some parameter regions.

Sensitivity needed to explore the seesaw mechanism is significantly lower than previous estimates.

CP-violating phases greatly influence the minimal detectable mixing angles.

Abstract

Seesaw mechanism constrains from below mixing between active and sterile neutrinos for fixed sterile neutrino masses. Signal events associated with sterile neutrino decays inside a detector at fixed target experiment are suppressed by the mixing angle to the power of four. Therefore sensitivity of experiments such as SHiP and DUNE should take into account minimal possible values of the mixing angles. We extend the previous study of this subject arXiv:1312.2887 to a more general case of non-zero CP-violating phases in the neutrino sector. Namely, we provide numerical estimate of minimal value of mixing angles between active neutrinos and two sterile neutrinos with the third sterile neutrino playing no noticeable role in the mixing. Thus we obtain a sensitivity needed to fully explore the seesaw type I mechanism for sterile neutrinos with masses below 2 GeV, and one undetectable sterile neutrino that is relevant for the fixed-target experiments. Remarkably, we observe a strong dependence of this result on the lightest active neutrino mass and the neutrino mass hierarchy, not only on the values of CP-violating phases themselves. All these effects sum up to push the limit of experimental confirmation of sterile-active neutrino mixing by several orders of magnitude below the results of arXiv:1312.2887 from $10^{-10}$ - $10^{-11}$ down to $10^{-12}$ and even to $10^{-20}$ in parts of parameter space; nonzero CP-violating phases are responsible for that.