Evidence of Single State Dominance in the Two-Neutrino Double-β Decay of 82Se with CUPID-0

We report on the measurement of the two-neutrino double beta decay of Se-82 performed for the first time with cryogenic calorimeters, in the framework of the CUPID-0 experiment. With an exposure of 9.95 kg yr of (ZnSe)-Se-82, we determine the two-neutrino double-beta decay half-life of Se-82 with an unprecedented precision level, T-1/2(2v) = [8 60 +/- 0.03(stat)(-0.13)(+0.19) (syst)] x 10(19) yr. The very high signal-to-background ratio, along 1/2 L with the detailed reconstruction of the background sources allowed us to identify the single state dominance as the underlying mechanism of such a process, demonstrating that the higher state dominance hypothesis is disfavored at the level of 5.5 sigma.