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

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.