Neutrinoless double-beta decay with massive scalar emission

Kfir Blum,Yosef Nir,Michal Shavit

doi:10.1016/j.physletb.2018.08.022

Abstract

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.

Highlights

Ld=5 = − Zαβ (H Lα)(H Lβ ), (3)Neutrinoless double beta (0ν2β) decay [1,2,3,4],(A, Z ) → (A, Z + 2) + 2e−, (1)is a lepton number violating process
A well known framework that leads to Eq (5) and to the decay mode 0ν2βφ is that of majoron models [8,9,10,11,12,13], where φ is the Goldstone boson related to the spontaneous breaking of the lepton number symmetry
This can occur if resonant s-channel scattering of a highenergy astrophysical neutrino with energy ν off the cosmic neutrino background (CνB) is possible, which in turn requires the mediator mass to match the center of mass energy (CME) of the collision, mφ =

Summary

Introduction

A well known framework that leads to Eq (5) and to the decay mode 0ν2βφ is that of majoron models [8,9,10,11,12,13], where φ is the Goldstone boson related to the spontaneous breaking of the lepton number symmetry. We have μ/ 2, where a technically natural hierarchy μ is responsible, at least in part, for the smallness of the neutrino mass In such case, a light scalar field could arise if we promote the inverse-seesaw parameter μ to a field φ with μ = φ. If the explicit lepton number violation (LNV) dominates the neutrino mass, the relation between G and mν is modified. 0ν2β(φ∗ → 2ν) cannot be constrained with current experiments, and we limit our attention to on-shell 0ν2βφ

Neutrinoless double-beta decay with massive scalar emission

Implications for scalar-mediated neutrino self-interactions

Conclusions