Probing Dark Sector Particles Coupling to Neutrinos with Double Beta Decay

TL;DR

The work investigates the sensitivity of current and future double beta decay experiments to a light Majoron-like scalar coupled to neutrinos and potentially dark-sector fermions. By constructing three simplified UV-complete models with $s$-channel scalar mediation, it derives the amplitudes for $\chi_1\chi_2\beta\beta$ and scalar-emission channels, including interference with SM $2\nu\beta\beta$ and the standard $0\nu\beta\beta$-like Majoron case, and computes the resulting electron-energy spectra using NMEs. The authors implement a rigorous statistical framework with Asimov data sets and nuisance parameter treatment for NMEs to project sensitivities for current experiments (GERDA II, KamLAND-Zen, NEMO-3) and future facilities (LEGEND-1000, CUPID, nEXO), finding that scalar-neutrino couplings down to $|a_{\nu}| \approx 2\times 10^{-6}$ can be probed for sub-MeV scalars, including off-shell scenarios above the $\beta\beta$ Q-value. They also map the viable parameter space against cosmological and laboratory constraints (BBN, CMB, kaon decays), highlighting complementary bounds and showing that double beta decay provides a powerful, independent laboratory probe of light scalars and dark-sector fermions in this mass range. Overall, the study demonstrates that targeted distortions and thresholds in the $2\nu\beta\beta$ spectrum can reveal new physics connected to neutrino properties and dark sectors, with strong synergy between laboratory searches and cosmological observations.

Abstract

Motivated by the observation of non-zero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta $(0νββ)$ decay. In all of these searches, a substantial amount of data on two-neutrino double beta $(2νββ)$ decay has been collected. In this work, we explore the sensitivity of current and future double beta decay experiments to a massive Majoron-like scalar particle coupled to neutrinos and potentially dark sector fermions, and compare their reach to the relevant cosmological constraints. On- and off-shell production of such scalar particles leads to characteristic distortions in the double beta decay electron spectrum. We investigate how these distortions would manifest in current and future double beta decay experiments, deriving the sensitivity to such scenarios. We project the reach of future experiments which can probe scalar-neutrino couplings of $|a_ν| \approx 2\times 10^{-6}$ for sub-MeV scalar particles and remain sensitive to off-shell production above the Q-value of double beta decay isotopes.

Figures (10)

• Figure 1: Feynman diagrams for neutrinoless double beta decay with scalar Majoron(-like) emission (left) and neutrinoless double beta decay with exotic fermion-pair emission ($2\chi\beta\beta$) via $s$-channel scalar exchange (right).
• Figure 2: Total decay width of the scalar particle $S$ as a function of its mass $m_S$ in the three different models considered. The active neutrino is considered massless whereas we take $m_{\chi_{1,2}} = 0.5$ MeV. In each model, all $a_i$ and $b_i$ couplings are set to unity. The active-sterile mixing in Model I is set to $\theta = 10^{-4}$.
• Figure 3: Differential distribution $d\Gamma/dT$ of $2\chi\beta\beta$ decay of $^{100}$Mo with respect to the total electron kinetic energy $T$ in Model I$^\prime$ for three different parameter sets: $(m_S, a_\nu, a_\chi) = (0.5~\text{MeV}, 3\times 10^{-3}, \sqrt{2})$ (blue), $(2.8~\text{MeV}, 7\times 10^{-2}, \sqrt{2})$ (green) and $(3.5~\text{MeV}, 2\times 10^{-1}, \sqrt{2})$ (red). All other couplings are set to zero and the $\chi$ are massless. For comparison, the scalar emission contributions $0\nu\beta\beta S$ are also shown, for the corresponding values of $m_S$ and $a_\nu$. The SM $2\nu\beta\beta$ and ordinary Majoron emission $0\nu\beta\beta$ (with $a_\nu = 3\times 10^{-3}$) are shown in grey. The relevant NMEs are given in Table \ref{['tab:isotopes']}.
• Figure 4: As Fig. \ref{['fig:spectra']}, but showing $d\Gamma^{2\chi}_S/dT$ for $m_S = 0.5$ MeV and $m_\chi = 0.1$ MeV (blue), 0.5 MeV (green) and 1.0 MeV (red). The non-zero couplings are $(a_\nu, a_\chi) = (3\times 10^{-3}, \sqrt{2})$ (blue), $(7\times 10^{-2}, \sqrt{2})$ (green) and $(3\times 10^{-1}, \sqrt{2})$ (red).
• Figure 5: Differential distribution $d\Gamma/dT$ of $2\nu\beta\beta$ decay of $^{100}$Mo with respect to the total electron kinetic energy $T$ including contributions from SM $2\nu\beta\beta$, $S$-mediated $2\nu_S\beta\beta$ and their interference (green). The scalar mass is $m_S = 2.8$ MeV and $a_\nu = 7\times 10^{-2}$ with all other couplings set to zero. For comparison, the scalar emission contribution $0\nu\beta\beta S$ (using the same parameters) and SM $2\nu\beta\beta$ are also shown. The relevant NMEs are given in Table \ref{['tab:isotopes']}.