The NEMO-3 detector, which had been operating in the Modane Underground Laboratory from 2003 to 2010, was designed to search for neutrinoless double $beta$ ($0 ubetabeta$) decay. We report final results of a search for $0 ubetabeta$ decays with $6.91
4$ kg of $^{100}$Mo using the entire NEMO-3 data set with a detector live time of $4.96$ yr, which corresponds to an exposure of 34.3 kg$cdot$yr. We perform a detailed study of the expected background in the $0 ubetabeta$ signal region and find no evidence of $0 ubetabeta$ decays in the data. The level of observed background in the $0 ubetabeta$ signal region $[2.8-3.2]$ MeV is $0.44 pm 0.13$ counts/yr/kg, and no events are observed in the interval $[3.2-10]$ MeV. We therefore derive a lower limit on the half-life of $0 ubetabeta$ decays in $^{100}$Mo of $T_{1/2}(0 ubetabeta)> 1.1 times 10^{24}$ yr at the $90%$ Confidence Level, under the hypothesis of light Majorana neutrino exchange. Depending on the model used for calculating nuclear matrix elements, the limit for the effective Majorana neutrino mass lies in the range $langle m_{ u} rangle < 0.33$--$0.62$ eV. We also report constraints on other lepton-number violating mechanisms for $0 ubetabeta$ decays.
We report the results of a search for the neutrinoless double-$beta$ decay (0$ ubetabeta$) of $^{100}$Mo, using the NEMO-3 detector to reconstruct the full topology of the final state events. With an exposure of 34.7 kg.y, no evidence for the 0$ ubet
abeta$ signal has been found, yielding a limit for the light Majorana neutrino mass mechanism of $T_{1/2}(0 ubetabeta)>1.1 times 10^{24}$ years (90% C.L.) once both statistical and systematic uncertainties are taken into account. Depending on the Nuclear Matrix Elements this corresponds to an upper limit on the Majorana effective neutrino mass of $< m_{ u} > < 0.3-0.9$ eV (90% C.L.). Constraints on other lepton number violating mechanisms of 0$ ubetabeta$ decays are also given. Searching for high-energy double electron events in all suitable sources of the detector, no event in the energy region [3.2-10] MeV is observed for an exposure of 47 kg.y.
