Neutrinoless double-electron capture

Abstract

Efforts to understand the character of the neutrino, and searches for physics beyond the standard model, motivate several ongoing experiments to detect neutrinoless double-beta decay. The complementary process of double-electron capture has received less attention. Currently the limits on capture measurements are not competitive with the limits on decay measurements. With a look to future experiments, this review covers the current status, emphasizes the significant enhancements that can occur when a resonance condition exists, and provides a road map for future progress.