Parametrization of Seesaw Models and Light Sterile Neutrinos

TL;DR

This paper introduces an exact parametrization for minimal Standard Model extensions with right-handed neutrinos, demonstrating that such simple models can replicate the best fit regions for sterile neutrino anomalies previously modeled with more complex scenarios.

Contribution

It provides a new exact parametrization of seesaw models with right-handed neutrinos, capturing all correlations and applicable beyond the seesaw approximation.

Findings

The parametrization includes all correlations among masses and mixings.

Minimal models with three right-handed neutrinos can reproduce 3+2 scenario fit regions.

The approach simplifies the analysis of sterile neutrino models.

Abstract

The recent recomputation of the neutrino fluxes from nuclear reactors relaxes the tension between the LSND and MiniBooNE anomalies and disappearance data when interpreted in terms of sterile neutrino oscillations. The simplest extension of the Standard Model with such fermion singlets is the addition of right-handed sterile neutrinos with small Majorana masses. Even when introducing three right-handed neutrinos, this scenario has less free parameters than the 3+2 scenarios studied in the literature. This begs the question whether the best fit regions obtained can be reproduced by this simplest extension of the Standard Model. In order to address this question, we devise an exact parametrization of Standard Model extensions with right-handed neutrinos. Apart from the usual 3x3 neutrino mixing matrix and the 3 masses of the lightest neutrinos, the extra degrees of freedom are encoded in another 3x3 unitary matrix and 3 additional mixing angles. The parametrization includes all the correlations among masses and mixings and is valid beyond the usual seesaw approximation. Through this parametrization we find that the best fit regions for the LSND and MiniBooNE anomalies in a 3+2 scenario can indeed be reproduced despite the smaller number of degrees of freedom.