Measurement of the $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ Decay Half-Life and Search for the $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ Decay of ${}^{116}$Cd with the NEMO-3 Detector

Abstract

The NEMO-3 experiment measured the half-life of the $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay and searched for the $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay of ${}^{116}$Cd. Using $410$ g of ${}^{116}$Cd installed in the detector with an exposure of $5.26$ y, ($4968\pm 74$) events corresponding to the $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay of ${}^{116}$Cd to the ground state of ${}^{116}$Sn have been observed with a signal to background ratio of about $12$. The half-life of the $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay has been measured to be $ T_{1/2}^{2ν}=[2.74\pm0.04\mbox{(stat.)}\pm0.18\mbox{(syst.)}]\times10^{19}$ y. No events have been observed above the expected background while searching for $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay. The corresponding limit on the half-life is determined to be $T_{1/2}^{0\hat{I}½}\ge 1.0\times 10^{23}$ y at the $90$ % C.L. which corresponds to an upper limit on the effective Majorana neutrino mass of $⟨m_{\hat{I}}½⟩\le 1.4-2.5$ eV depending on the nuclear matrix elements considered. Limits on other mechanisms generating $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay such as the exchange of R-parity violating supersymmetric particles, right-handed currents and majoron emission are also obtained.