Pinpointing the Mechanism of Neutrinoless Weak Decays with Positrons

Neutrinoless double beta decay is the flagship laboratory probe of a Majorana contribution to the neutrino mass. However, besides the standard mass mechanism other higher- dimensional lepton number-violating interactions can enter, or even dominate, this process. The corresponding positron-emitting neutrinoless modes, such as electron capture or double-positron emission, have long been considered out of reach experimentally, due to their naturally smaller rates. Recently, innovative detector technologies as used in the proposed NuDoubt++ experiment are changing the game. In this work, we explore how a positron- and electron-mode detector can be complementary in the search for new physics. Assuming an observation of neutrinoless double beta decay, we predict the expected discovery half-life for the positron-modes. Using half-life ratios, especially between the double beta and electron capture modes, we demonstrate how underlying long-range interactions can be distinguished, in particular to identify a purely right-handed leptonic current.