New measurement of double beta decays of ${}^{100}$Mo to excited states of ${}^{100}$Ru with the CUPID-Mo experiment

Abstract

The CUPID-Mo experiment, located at Laboratoire Souterrain de Modane (France), was a demonstrator experiment for CUPID. It consisted of an array of 20 Li${}_2^{100}$MoO${}_4$ (LMO) calorimeters each equipped with a Ge light detector (LD) for particle identification. In this work, we present the result of a search for two-neutrino and neutrinoless double beta decays of ${}^{100}$Mo to the first 0${}^{+}$ and $2^{+}$ excited states of ${}^{100}$Ru using the full CUPID-Mo exposure (2.71 kg$\times$yr of LMO). We measure the half-life of $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay to the $0_1^{+}$ state as $T_{1/2}^{2\hat{I}½\to 0_1^{+}}=7.5\pm 0.8\text{(stat.)}{}_{-0.3}^{+0.4}\text{(syst.)})\times 10^{20}\mathrm{yr}$. The bolometric technique enables measurement of the electron energies as well as the gamma rays from nuclear de-excitation and this allows us to set new limits on the two-neutrino decay to the $2_1^{+}$ state of $T^{2ν\rightarrow 2_1^+}_{1/2}>4.4\times 10^{21} \ \mathrm{yr} \ \text{(90 % c.i.)}$ and on the neutrinoless modes of $T_{1/2}^{0ν\rightarrow 2_1^+}>2.1\times10^{23} \ \mathrm{yr}\ \text{(90 % c.i.)}$, $T_{1/2}^{0ν\rightarrow 0_1^+}>1.2\times10^{23} \ \mathrm{yr}\ \text{(90 % c.i.)}$. Information on the electrons spectral shape is obtained which allows us to make the first comparison of the single state (SSD) and higher state (HSD) $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay models for the $0_1^{+}$ excited state of ${}^{100}$Ru.

22 pages, 15 figures