Final Result on the Neutrinoless Double Beta Decay of ${}^{82}\mathrm{Se}$ with CUPID-0

CUPID-0, an array of $\mathrm{Zn}{}^{82}\mathrm{Se}$ cryogenic calorimeters, was the first medium-scale demonstrator of the scintillating bolometers’ technology. The first project phase (March 2017–December 2018) allowed the most stringent limit on the neutrinoless double beta decay half-life of the isotope of interest, ${}^{82}\mathrm{Se}$, to be set. After a six month long detector upgrade, CUPID-0 began its second and last phase (June 2019–February 2020). In this Letter, we describe the search for neutrinoless double beta decay of ${}^{82}\mathrm{Se}$ with a total exposure (phase $\mathrm{I}+\mathrm{II}$) of $8.82\mathrm{kg}{\mathrm{yr}}^{-1}$ of isotope. We set a limit on the half-life of ${}^{82}\mathrm{Se}$ to the ground state of ${}^{82}\mathrm{Kr}$ of ${\mathrm{T}}_{1/2}^{0\nu }({}^{82}\mathrm{Se})>4.6\times {10}^{24}\mathrm{yr}$ (90% credible interval), corresponding to an effective Majorana neutrino mass $m_{\beta \beta }<(263–545)\mathrm{meV}$. We also set the most stringent lower limits on the neutrinoless decays of ${}^{82}\mathrm{Se}$ to the $0_1^{+}$, $2_1^{+}$, and $2_2^{+}$ excited states of ${}^{82}\mathrm{Kr}$, finding $1.8\times {10}^{23}\mathrm{yr}$, $3.0\times {10}^{23}\mathrm{yr}$, and $3.2\times {10}^{23}\mathrm{yr}$ (90% credible interval) respectively.

Figure 1

Physics spectrum with a ${}^{82}\mathrm{Se}$ exposure of $8.82\mathrm{kg}\times \mathrm{yr}$ ($16.59\mathrm{kg}\times \mathrm{yr}$ in ZnSe). Gray: spectrum of pulses selected according to their bolometric quality, with the additional condition that a single ZnSe detector triggered the event. Orange: same spectrum after rejecting $\alpha$ events. Blue: events surviving the delayed coincidence veto with potential ${}^{212}\mathrm{Bi}$ parents. Green line: 90% C.I. Bayesian upper limit (see text) superimposed over the measured background.