Probing Dark Sector Particles Coupling to Neutrinos with Double Beta Decay

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 $(0\nu\beta\beta)$ decay. In all of these searches, a substantial amount of data on two-neutrino double beta $(2\nu\beta \beta)$ 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 a scalar leads to characteristic distortions in the emitted electron spectrum. We investigate how these distortions manifest in current and future double beta decay experiments, deriving the sensitivity to such a scenario. We project the reach of future experiments which can probe scalar-neutrino couplings of $|a_\nu| \approx 2\times 10^{-6}$ for sub-MeV scalar particles and remain sensitive to off-shell production above the Q-value of double beta isotopes.