On neutrinoless double beta decay in the $\nu$MSM

TL;DR

This paper investigates how the heavy neutral leptons in the $ u$MSM model can enhance the neutrinoless double beta decay rate, especially under inverted neutrino mass hierarchy, challenging previous assumptions of destructive interference.

Contribution

It demonstrates that the effective mass for $0 u eta eta$ decay can be increased due to the constructive contribution of heavy neutrinos when their mass difference is larger and the neutrino mass ordering is inverted.

Findings

Effective mass can exceed active neutrino contribution.

Constructive interference occurs with larger mass differences.

Potential for detection in upcoming $0 u eta eta$ experiments.

Abstract

We consider the neutrinoless double beta ($0\nu \beta \beta$) decay in the so-called $\nu$MSM, in which three right-handed neutrinos with masses below the electroweak scale are additionally introduced to the Standard Model. In this model there appear three heavy neutral leptons $N_1$, $N_2$, and $N_3$ corresponding to right-handed neutrinos. It has been known that the lightest one $N_1$ with keV mass, which is a candidate for dark matter, gives a negligible contribution to the $0 \nu \beta \beta$ decay. By contrast, the heavier ones $N_2$ and $N_3$, which are responsible to the seesaw mechanism of neutrino masses and baryogenesis, give the destructive contribution (compared with one from active neutrinos). This is because their mass degeneracy at high precision has been assumed, which is expected by analytical studies of baryogengesis. In this analysis, we find that the effective mass of the $0\nu \beta \beta$ decay becomes larger than one from active neutrinos due to the $N_2$ and $N_3$ constructive contribution when the mass difference becomes larger and the mass ordering of active neutrinos is inverted. Such a possibility will be explored by the current and near future experiments of the $0 \nu \beta \beta$ decay.