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

Neutrinoless double beta decay (0 nu beta beta) 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 (2 nu beta beta) is allowed by the standard model and has been measured in numerous experiments. In this Letter, we report a measurement of 2 nu beta beta decay half-life of Mo-100 to the ground state of Ru-100 of [7.07 +/- 0.02(stat) +/- 0.11(syst)+/- x 10(18) yr by the CUPID-Mo experiment. With a relative precision of +/- 1.6% this is the most precise measurement to date of a 2 nu beta beta decay rate in Mo-100. 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 xi(3,1) = 0.45 +/- 0.03(stat) +/- 0.05(syst) 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 2 nu beta beta decay.