Neutrinoless double-beta decay with massive scalar emission

TL;DR

This paper investigates the impact of a massive scalar emission in neutrinoless double-beta decay, revealing that constraints weaken significantly for scalar masses near the reaction energy, with implications for neutrino detection experiments.

Contribution

It introduces the analysis of neutrinoless double-beta decay with a massive scalar, a scenario not previously considered, and estimates the resulting constraints on such models.

Findings

Constraints weaken for scalar masses above the reaction threshold.

Kinematic phase space suppression reduces the decay signal.

Implications for high-energy neutrino telescope experiments.

Abstract

Searches for neutrino-less double-beta decay ($0\nu2\beta$) place an important constraint on models where light fields beyond the Standard Model participate in the neutrino mass mechanism. While $0\nu2\beta$ experimental collaborations often consider various massless majoron models, including various forms of majoron couplings and multi-majoron final-state processes, none of these searches considered the scenario where the "majoron" $\phi$ is not massless, $m_\phi\sim$~MeV, of the same order as the $Q$-value of the $0\nu2\beta$ reaction. We consider this parameter region and estimate $0\nu2\beta\phi$ constraints for $m_\phi$ of order MeV. The constraints are affected not only by kinematical phase space suppression but also by a change in the signal to background ratio characterizing the search. As a result, $0\nu2\beta\phi$ constraints for $m_\phi>0$ diminish significantly below the reaction threshold. This has phenomenological implications, which we illustrate focusing on high-energy neutrino telescopes. Our results motivate a dedicated analysis by $0\nu2\beta$ collaborations, analogous to the dedicated analyses targeting massless majoron models.