Neutrinoless double-beta decay with massive scalar emission

Searches for neutrino-less double-beta decay (0ν2β) place an important constraint on models where light fields beyond the Standard Model participate in the neutrino mass mechanism. While 0ν2β 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” ϕ is not massless, mϕ∼ MeV, of the same order as the Q-value of the 0ν2β reaction. We consider this parameter region and estimate 0ν2βϕ constraints for mϕ 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 charachterizing the search. As a result, 0ν2βϕ constraints for mϕ>0 diminish significantly below the reaction threshold. This has phenomenological implications, which we illustrate focusing on high-energy neutrino telescopes. The spectral shape of high-energy astrophysical neutrinos could exhibit features due to resonant νν→ϕ→νν scattering. Such features fall within the sensitivity range of IceCube-like experiments, if mϕ is of order MeV, making 0ν2βϕ a key complimentary laboratory constraint on the scenario. Our results motivate a dedicated analysis by 0ν2β collaborations, analogous to the dedicated analyses targeting massless majoron models.