The Cryogenic Underground Observatory for Rare Events (“CUORE”) is an experiment located underground at the Laboratori Nationali del Gran Sasso (Gran Sasso National Laboratory – “LNGS”) in Italy. The principal goal of CUORE is to search for the Neutrino-Less Double Beta Decay (NLDBD) of Tellurium-130 (130Te). NLDBD is hypothetical and yet-to-be-observed nuclear decay that involves the emission of two electrons without the corresponding neutrinos. In addition to the creation of matter (the two electrons), which could be a possible explanation for the observed matter-antimatter asymmetry in the universe, the observation of NLDBD would demonstrate that the neutrino is a so-called Majorana Fermion where the neutrino and antineutrino are identical particles. CUORE is one of several international experiments searching for the NLDBD of various isotopes.
CUORE, operating since 2017, consists of 988 Tellurium-Oxide (TeO2) crystal cryogenic calorimeters with a total mass of 742 kilograms. A custom 3He/4He dilution refrigerator cools these crystals to a temperature of about 15 millikelvins, making the detector volume the coldest known cubic meter in the known universe. CUORE measures the energy deposited into each crystal at high resolution by employing a sensitive thermometer to detect the slight temperature rise. As of this writing, CUORE has not observed 130Te NLDBD, but it set the most stringent limits on this process [Nature 604, 53 (2022)].
Cuore Upgrade with Particle IDentification (“CUPID”), a proposed upgrade to CUORE, is one of the Ton-Scale experiments capable of discovering NLDBD in the parameter space’s so-called inverted mass ordering region. CUPID will employ Lithium molybdate (Li2MoO4) crystals (instead of TeO2), which will improve the sensitivity to NLDBD due to the increased energy released by Molybdenum-100 (100Mo). In addition to measuring heat (i.e., temperature rise), CUPID will also measure the scintillation light from NLDBD events in Li2MoO4, giving CUPID the power to employ particle identification to reject background events. The CUPID collaboration is currently preparing to deploy CUPID Stage I.
