Final results on $${}^\mathbf{82 }{\hbox {Se}}$$82Se double beta decay to the ground state of $${}^\mathbf{82 }{\hbox {Kr}}$$82Kr from the NEMO-3 experiment

Using data from the NEMO-3 experiment, we have measured the two-neutrino double beta decay (\(2\nu \beta \beta \)) half-life of \(^{82}\)Se as \(T_{\smash {1/2}}^{2\nu } \!=\! \left[ 9.39 \pm 0.17\left( \text{ stat }\right) \pm 0.58\left( \text{ syst }\right) \right] \times 10^{19}\) y under the single-state dominance hypothesis for this nuclear transition. The corresponding nuclear matrix element is \(\left| M^{2\nu }\right| = 0.0498 \pm 0.0016\). In addition, a search for neutrinoless double beta decay (\(0\nu \beta \beta \)) using 0.93 kg of \(^{82}\)Se observed for a total of 5.25 y has been conducted and no evidence for a signal has been found. The resulting half-life limit of \(T_{1/2}^{0\nu } > 2.5 \times 10^{23} \,\text{ y } \,(90\%\,\text{ C.L. })\) for the light neutrino exchange mechanism leads to a constraint on the effective Majorana neutrino mass of \(\langle m_{\nu } \rangle < \left( 1.2{-}3.0\right) \,\text{ eV }\), where the range reflects \(0\nu \beta \beta \) nuclear matrix element values from different calculations. Furthermore, constraints on lepton number violating parameters for other \(0\nu \beta \beta \) mechanisms, such as right-handed currents, majoron emission and R-parity violating supersymmetry modes have been set.