Scalarful double beta decay

TL;DR

This work investigates neutrinoless double beta decay with scalar emission ($0νββφ$) in an EFT setting, connecting scalar couplings to standard $0νββ$ NMEs via mass and mixing dependent amplitudes. It derives the mass dependent amplitudes $A^φ$ for active, sterile, and mixed neutrino couplings and computes the associated phase-space factors $Ω^φ$ and angular observables, for scalars with $m_φ$ below and beyond the isotope Q-value. A rigorous frequentist analysis using Asimov data and multi-observable schemes yields constraints on the effective scalar–neutrino coupling $g_{ee}$ across isotopes such as $^{136}$Xe and $^{100}$Mo, including off-shell and right-handed-current scenarios. The results show that NMEs uncertainties dominate the limits, reveal a funnel in sterile-neutrino cases due to sign changes, and indicate that $^{136}$Xe near the Q-value provides the strongest current sensitivity, guiding future experimental and theoretical work.

Abstract

We revisit scalar emissions in double beta decays of nuclei, often discussed in the context of Majoron models, in light of the latest developments on the study of neutrinoless double beta decay amplitudes from an effective field theory approach. The sensitivity of double beta decay experiments to this process is assessed through an analysis of spectral shapes, and the study is extended to massive scalars, scalars coupling to sterile neutrinos, and exotic right-handed effective couplings.

Figures (14)

• Figure 1: Leading order contributions to $0\nu\beta\beta\phi$. The grey circles denote the effective weak vertex, and the black circles indicate the bound states of nucleons. A neutrino is exchanged between the nucleons, which radiates the scalar $\phi$.
• Figure 2: The summed electron energy (left) and single electron energy (right) spectra for $2\nu\beta\beta$ and $0\nu\beta\beta$ with scalar emission for various scalar masses for ${}^{136}$Xe. The $2\nu\beta\beta$ and $m_\phi =0$ distributions are normalised to unity, while the rest are normalised to the $m_\phi=0$ distribution.
• Figure 3: The angular correlation for $2\nu\beta\beta$ and $0\nu\beta\beta$ with scalar emission for various scalar masses for ${}^{136}$Xe. The $2\nu\beta\beta$ and $m_\phi =0$ distributions are normalised to unity, while the rest are normalised to the $m_\phi=0$ distribution.
• Figure 4: Constraints on the scalar-active neutrino coupling as a function of the scalar mass, for different observables for $^{100}$Mo.
• Figure 5: Constraints on the scalar-active neutrino coupling from the summed electron energy spectrum, for different isotopes.