Final Result of CUPID-0 Phase-I in the Search for the Se 82 Neutrinoless Double- β Decay

CUPID-0 is the first pilot experiment of CUPID, a next-generation project for the measurement of neutrinoless double beta decay (0 nu DBD) with scintillating bolometers. The detector, consisting of 24 enriched and 2 natural ZnSe crystals, has been taking data at Laboratori Nazionali del Gran Sasso from June 2017 to December 2018, collecting a Se-82 exposure of 5.29 kg x yr. In this Letter we present the phase-I results in the search for O nu DBD. We demonstrate that the technology implemented by CUPID-0 allows us to reach the lowest background for calorimetric experiments: (3.5(-0.9)(+1.0)) x 10(-3) counts/ (keV kg yr). Monitoring 3.88 x 10(25) Se-82 nuclei x yr we reach a 90% credible interval median sensitivity of T-1/2(0 nu) > 5.0 x 10(24) yr and set the most stringent limit on the half-life of Se-82 O nu DBD: T-1/2(0 nu) > 3.5 x 10(24) yr (90% credible interval), corresponding to m(beta beta) < (311-638) meV depending on the nuclear matrix element calculations.