CUPID-Mo is a pilot experiment searching for neutrinoless double beta decay of the nucleus Mo-100. The CUPID-Mo technology consists of Li2MoO4 scintillating bolometers containing the nucleus of interest, developed in the context of the LUMINEU project

The neutrinoless double beta decay signature is a peak in the sum energy spectrum of the two emitted electrons. In Mo-100, this peak is expected at 3034 keV, well above the bulk of the gamma environmental radioactivity. The remaining dominant source of background in this region are alpha particles, that can be rejected in scintillating bolometers thanks to the simultaneous measurement of scintillation and heat.  In fact, alphas emit much less light than betas for the same heat energy. These features allow the CUPID-Mo technology to approach a zero background condition, even for large scale experiments.

CUPID-Mo consists of an array of twenty ∼ 200 g Li2MoO4 detectors enriched to ∼ 97 % in Mo-100 with a total mass of 4.16 kg of Li2MoO4 and 2.26 kg of Mo-100. These detectors were operated at ∼ 20 mK in the EDELWEISS cryostat at the Laboratoire Souterrain de Modane in France. This technique allows for a very high detection efficiency (∼ 75 % for neutrinoless double beta decay to g.s.) due to the detector containing the source, and excellent energy resolution (σ ∼ 0.1%) at 3 MeV. In addition, CUPID-Mo employed a dual readout with twenty cryogenic light detectors consisting of Ge wafers also operated as bolometers. Each Li2MoO4 crystal is assembled into an independent detector module with a Ge light detector. The individual detector modules are arranged in an array of five towers so that each Li2MoO4 detector faces two light detectors (apart from those on the top floor which only have a lower light detector).

CUPID-Mo took data between 2019 and 2020, collecting a total exposure of 2.71 kg-yr of Li2MoO4 and achieving remarkable physics and technical result. We mention the most stringent bound worldwide on the neutrinoless double beta decay half-life of Mo-100: 1.8 x 10^24 years, corresponding to limits on the effective Majorana neutrino mass in the range 280-490 meV. A CUPID-Mo precursor with much less mass has established the most precise measurement of the two neutrino double beta decay of Mo-100. This result is expected to be improved by the full CUPID-Mo data set. The analysis is still in progress and other important results are expected on various hypothetical exotic processes.

Thanks to the reliability of the detector operation, the excellent energy resolution, the particle identification capability and the radiopurity of the Li2MoO4 enriched crystals demonstrated by CUPID-Mo, the technology of Li2MoO4 scintillating bolometers has been selected for the next-generation CUPID experiment.