Abstract: Searches for neutrinoless double beta decay are almost universally interpreted through a single observable, the half-life, and a single mechanism, the exchange of a light Majorana neutrino. Neither condition gives the full story.
In the low-energy effective theory (LEFT) of lepton number violation, leading structures beyond the dimension-three mass operator arise at dimension six, where five independent long-range interactions contribute to the amplitude. Each carries a distinct chiral structure and therefore a distinct dependence on the outgoing lepton kinematics. In neutrinoless double beta decay (electron-emitting modes) these differences are largely washed out: the relevant phase-space factors scale similarly across candidate isotopes, so half-life ratios between isotopes vary by only a factor of a few. However, there are similar, but suppressed, decays which emit positrons. In this work, we show that these positron-emitting channels break this degeneracy. We consider positron-emitting channels in the available isotopes for all effective couplings, studying the full interference structure, and evaluate whether the combination of positron modes and electron mode is useful. Although the positron modes will not race neutrinoless double beta decay experiments to discovery; they could tell us what a discovery means.