Probing dark sector particles coupling to neutrinos with double beta decay

Motivated by the observation of nonzero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mo stretchy="false">(</a:mo> <a:mn>0</a:mn> <a:mi>ν</a:mi> <a:mi>β</a:mi> <a:mi>β</a:mi> <a:mo stretchy="false">)</a:mo> </a:math> decay. In all of these searches, a substantial amount of data on two-neutrino double beta <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mo stretchy="false">(</e:mo> <e:mn>2</e:mn> <e:mi>ν</e:mi> <e:mi>β</e:mi> <e:mi>β</e:mi> <e:mo stretchy="false">)</e:mo> </e:math> 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 <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mo stretchy="false">|</i:mo> <i:msub> <i:mi>a</i:mi> <i:mi>ν</i:mi> </i:msub> <i:mo stretchy="false">|</i:mo> <i:mo>≈</i:mo> <i:mn>2</i:mn> <i:mo>×</i:mo> <i:msup> <i:mn>10</i:mn> <i:mrow> <i:mo>−</i:mo> <i:mn>6</i:mn> </i:mrow> </i:msup> </i:math> for sub-MeV scalar particles and remain sensitive to off-shell production above the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>Q</m:mi> </m:math> -value of double beta isotopes.