This so-called "double beta" radioactive decay is characterized by the emission of two electrons and two antineutrinos. If it took place without the emission of neutrinos (or antineutrinos), physicists would deduce that the neutrinos mix with their antiparticles, and would thus have clues to help determine their absolute masses. They would also understand the origin of their minuscule masses and the disappearance of antimatter in our Universe.

Neutrinoless double beta decay (0νββ) has an extremely clear and unambiguous signature. Scientists need to identify a peak in the total energy spectrum left in the detector by the two emitted electrons. The position of this peak is known to an accuracy of more than 0.1%.

However, events as rare as 0νββ require "zero" background noise. Indeed, the targeted radioactivity does not exceed 10^-13 Bq/g, whereas our daily radioactivity is counted in becquerels (Bq/g)! This is why the experiments are organized in underground laboratories sheltered from cosmic or telluric radiation, under several hundred meters of rock.

To go even further with noise reduction, researchers have adopted "scintillating" bolometers capable of measuring not only the subtle excess heat that signals radioactive decay, but also the light emission that it induces by scintillation. The source, molybdenum 100 (¹⁰⁰Mo), is embedded in a crystal that acts as both a heat detector (bolometer) and a scintillator. This concept makes it possible to combine a drastic reduction in the surrounding radioactive background with the exceptional energy resolution and efficiency of bolometers.

More concretely, the detector for the CUPID-Mo (CUORE Upgrade with Particle Identification-Mo) experiment at the Modane Underground Laboratory consists of 20 Li₂MoO₄ crystals enriched in ¹⁰⁰Mo, each weighing about 210 grams, i.e. 2.264 kg of ¹⁰⁰Mo. The detector has accumulated more than one year's worth of data between March 2019 and April 2020.

Through recognition of the dominant background noise (the α particles) and excellent "radio-purity" levels, the scientists at CUPID-Mo were able to attain zero background noise for their 0νββ analysis. The level achieved is already significantly better than that obtained in the most state-of-the-art international bolometric experiment, CUORE (Cryogenic Underground Observatory for Rare Events), located at Gran Sasso (Italy).

Thanks to this zero background noise, an efficient operating cycle and an excellent analysis efficiency (around 90%), the researchers were able to achieve a world record with what was only intended to be a technology demonstrator for the future CUPID experiment meant to succeed CUORE.

They thus set a new worldwide limit for the detection of the 0νββ signature in ¹⁰⁰Mo: the half-life obtained (1.4x10²⁴ years) exceeds the previous one (1.1x10²⁴ years) obtained by the international collaboration Nemo3 (Neutrino Ettore Majorana Experiment), with a longer experiment duration and a more massive radioactive source.