Measurement of the 2νββ Decay Rate and Spectral Shape of 100Mo from the CUPID-Mo Experiment

Neutrinoless double beta decay (Formula Presented) is a yet unobserved nuclear process that would demonstrate Lepton number violation, a clear evidence of beyond standard model physics. The process two neutrino double beta decay (Formula Presented) is allowed by the standard model and has been measured in numerous experiments. In this Letter, we report a measurement of Formula Presented decay half-life of Formula Presented to the ground state of Formula Presented of Formula Presented by the CUPID-Mo experiment. With a relative precision of Formula Presented this is the most precise measurement to date of a Formula Presented decay rate in Formula Presented. In addition, we constrain higher-order corrections to the spectral shape, which provides complementary nuclear structure information. We report a novel measurement of the shape factor Formula Presented based on a constraint on the ratio of higher-order terms from theory, which can be reliably calculated. This is compared to theoretical predictions for different nuclear models. We also extract the first value for the effective axial vector coupling constant obtained from a spectral shape study of Formula Presented decay.