$^{100}$Mo-enriched Li$_2$MoO$_4$ scintillating bolometers for $0ν 2β$ decay search: from LUMINEU to CUPID-0/Mo projects

Abstract: A scintillating bolometer technology based on $^{100}$Mo-enriched lithium molybdate (Li$2$$^{100}$MoO$_4$) crystals has been developed by LUMINEU to search for neutrinoless double-beta ($0\nu 2\beta$) decay of $^{100}$Mo. The results of several low temperature tests at underground environments have proved the reproducibility of high detector performance and crystal radiopurity: in particular $\sim$5--6~keV FWHM energy resolution and at least 9$\sigma$ rejection of $\alpha$'s in the vicinity of the $0\nu 2\beta$ decay of $^{100}$Mo (3034 keV) and below 10~$\mu$Bq/kg bulk activity of $^{228}$Th and $^{226}$Ra. A modest acquired exposure (0.1~kg$\times$yr) is a limiting factor of the LUMINEU experiment sensitivity to the $0\nu 2\beta$ decay half-life of $^{100}$Mo ($T{1/2}$ $\geq$ 0.7$\times$10$^{23}$ yr at 90\% C.L.), however the two-neutrino $2\beta$ decay has been measured with the best up to-date accuracy, $T_{1/2}$ = $\left[6.92 \pm 0.06(\mathrm{stat.}) \pm 0.36(\mathrm{syst.})\right] \times 10^{18}$ yr. The applicability of the LUMINEU technology for a tonne-scale $0\nu 2\beta$ decay bolometric project CUPID is going to be demonstrated by the CUPID-0/Mo experiment with $\sim$5~kg of $^{100}$Mo embedded in forty 0.2~kg Li$_2$$^{100}$MoO$_4$ scintillating bolometers. A first phase of the experiment with twenty Li$_2$$^{100}$MoO$_4$ detectors is in preparation at the Modane underground laboratory (France) to start by the end of 2017.