The minimal 3+2 neutrino model versus oscillation anomalies

TL;DR

This paper investigates a minimal 3+2 neutrino model with two singlet fermions, demonstrating it can better fit oscillation anomalies than the standard model with fewer parameters, and introduces a new parametrization for seesaw models.

Contribution

It introduces a new parametrization of seesaw models applicable when higher order corrections are significant, and shows the minimal 3+2 model fits oscillation anomalies effectively with fewer parameters.

Findings

The minimal 3+2 model can fit oscillation anomalies better than the standard 3-neutrino model.

The model favors a normal hierarchy and a large reactor angle.

It employs a new parametrization extending Casas-Ibarra for regimes with significant higher order corrections.

Abstract

We study the constraints imposed by neutrino oscillation experiments on the minimal extension of the Standard Model that can explain neutrino masses, which requires the addition of just two singlet Weyl fermions. The most general renormalizable couplings of this model imply generically four massive neutrino mass eigenstates while one remains massless: it is therefore a minimal 3+2 model. The possibility to account for the confirmed solar, atmospheric and long-baseline oscillations, together with the LSND/MiniBooNE and reactor anomalies is addressed. We find that the minimal model can fit oscillation data including the anomalies better than the standard $3\nu$ model and similarly to the 3+2 phenomenological models, even though the number of free parameters is much smaller than in the latter. Accounting for the anomalies in the minimal model favours a normal hierarchy of the light states and requires a large reactor angle, in agreement with recent measurements. Our analysis of the model employs a new parametrization of seesaw models that extends the Casas-Ibarra one to regimes where higher order corrections in the light-heavy mixings are significant.