Neutrinoless double beta decay rates and the $3 + 2$ scenario

TL;DR

This paper examines neutrinoless double beta decay within a $3 + 2$ neutrino model, highlighting the importance of ultrasoft contributions and proposing a new parametrization that differs from standard approaches, especially for light neutrinos.

Contribution

It introduces a new effective parametrization for decay amplitudes considering ultrasoft effects and explores the testability of a $3 + 2$ sterile neutrino model.

Findings

Significant differences in decay amplitude predictions for light neutrinos.

Ultrasoft contributions are crucial in the light-to-medium mass range.

The $3 + 2$ model offers testable predictions for neutrinoless double beta decay.

Abstract

The possible Majorana nature of neutrinos leads to lepton-number-violating effects such as neutrinoless double beta decay. The standard study of this process involves mass-dependent matrix elements which, although easy to use, might be missing important effects, especially in the light neutrino regime where the ultrasoft contributions become important. A fresh look at the different momentum regions leads us to an effective practical parametrisation for the decay amplitude that can show significant differences in the light-to-medium mass range of neutrinos compared to the standard parametrisation. As a concrete realisation of a UV model leading to Majorana neutrinos, the testability of a $3 + 2$ model with two sterile neutrinos is discussed.