Positron-Emitting and Electron-Capturing Double-Beta Processes in the Standard Model and Beyond

This research paper investigates positron-emitting and electron-capturing double-beta-decay modes as complementary probes to traditional double beta decay. It analyzes candidate isotopes like ${}^{78}$Kr, ${}^{106}$Cd, and ${}^{124}$Xe, providing nuclear matrix elements and phase-space factors for both neutrinoful and neutrinoless modes. The study also interprets projected sensitivities for neutrinoless channels in terms of SMEFT operators, indicating sensitivity to lepton-number-violating new-physics scales.