Final results on the $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay half-life limit of ${}^{100}$Mo from the CUPID-Mo experiment

Abstract

The CUPID-Mo experiment to search for 0$\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay in ${}^{100}$Mo has been recently completed after about 1.5 years of operation at Laboratoire Souterrain de Modane (France). It served as a demonstrator for CUPID, a next generation 0$\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay experiment. CUPID-Mo was comprised of 20 enriched Li${}_2$${}^{100}$MoO${}_4$ scintillating calorimeters, each with a mass of $\sim$ 0.2 kg, operated at $\sim$20 mK. We present here the final analysis with the full exposure of CUPID-Mo (${}^{100}$Mo exposure of 1.47 kg$\times$yr) used to search for lepton number violation via 0$\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay. We report on various analysis improvements since the previous result on a subset of data, reprocessing all data with these new techniques. We observe zero events in the region of interest and set a new limit on the ${}^{100}$Mo 0$\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay half-life of $T_{1/2}^{0\hat{I}½}>1.8\times 10^{24}$ year (stat.+syst.) at 90% CI. Under the light Majorana neutrino exchange mechanism this corresponds to an effective Majorana neutrino mass of $⟨m_{{\hat{I}}^2{\hat{I}}^2}⟩<(0.28$--$0.49)$ eV, dependent upon the nuclear matrix element utilized.