CROSS braves the cold to probe the true nature of neutrinos

Since late fall, the CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) experiment has officially begun its data collection phase. Installed at the Laboratorio Subterráneo de Canfranc in Spain, this experimental demonstrator, led by a team from IJCLab with significant collaboration from Irfu (CEA), aims to explore one of the rarest nuclear processes ever considered: double beta decay without neutrino emission.

Never observed to date, this hypothetical decay is a privileged gateway to physics beyond the Standard Model. “Its discovery would show that the neutrino is its own antiparticle, a particle known as a Majorana particle,” explains Andrea Giuliani, researcher at IJCLab and scientific coordinator of CROSS. Such a property would have major consequences, not only for particle physics but also for cosmology. It would shed light on the origin of neutrino mass and the asymmetry between matter and antimatter in the Universe."

In the “standard” version of double beta decay, already observed for several nuclei, two neutrons simultaneously transform into two protons, emitting two electrons and two neutrinos. This process is extremely rare—with half-lives exceeding 10¹⁸ years—but perfectly consistent with the known laws of physics. However, some theoretical models also predict a decay without neutrino emission, in which all the energy of the decay is transferred to the two electrons. Observing such a decay would reveal that neutrinos and antineutrinos are one and the same, a hypothesis formulated as early as 1937 by physicist Ettore Majorana.

CROSS will significantly improve sensitivity to the half-life of double beta decay without neutrino emission in a very promising isotope, molybdenum-100. When two neutrons in this nucleus transform into protons, changing the nucleus into ruthenium-100, the difference in mass between the initial and final states results in an energy of nearly 3 MeV, carried entirely by electrons in the case of neutrino-less decay. “This would manifest itself as a very sharp peak in the electron energy spectrum, unlike neutrino-less decay, which produces a continuous spectrum,” points out Andrea Giuliani. The challenge is therefore to measure the energy of the electrons with extreme precision, while eliminating countless sources of background noise.

To achieve this, CROSS has developed a complex experimental principle. First, as in most experiments detecting double beta decay without neutrino emission, the detector is both the source and the sensor of the event being sought. The experiment is based on 36 lithium molybdate crystals, 32 of which are enriched to 97% molybdenum-100, each weighing approximately 300 grams. These crystals, cooled to approximately 20 millikelvins, record the tiny temperature rise caused by the energy deposited by nuclear decay. At these extreme temperatures, energy on the order of MeV is sufficient to produce a thermal signal that can be measured by doped germanium thermometers attached to each crystal.

An additional advantage of CROSS, which builds on an idea tested with the CUPID-Mo experiment five years ago, is the combination of this thermal measurement with light detection. Lithium molybdate crystals are also scintillating: a very small fraction of the deposited energy is emitted in the form of photons. Opposite each crystal, an independent light detector measures this emission. The ratio between heat and light makes it possible to identify the nature of the particle responsible for the event and to effectively eliminate interactions due to alpha particles, the main source of background noise in this type of experiment.

But what makes CROSS unique is a major technological improvement in light detectors, also adopted by the CUPID collaboration following developments carried out at IJCLab: to further amplify the extremely weak light signal, CROSS uses an innovative device based on the Neganov-Trofimov-Luke effect. By applying an electric field in germanium light detectors, the energy deposited by photons is converted into an amplified thermal signal, greatly improving the ability to distinguish rare events from accidental pileups. This discrimination is essential because the energy signal emitted by two simultaneous ordinary beta decays would be indistinguishable from the signal expected for double beta decay without neutrino emission.

The detectors are organized into three towers, installed in a cryostat surrounded by shielding and supplemented by a cosmic muon veto. The latter device, which overlooks the apparatus, captures and filters out any cosmic muons that could enrich the background noise and blur the signal during analysis. It is to protect themselves from these particles emitted into the atmosphere that high-sensitivity experiments such as CROSS take refuge deep inside mountains. However, the modest depth of the Canfranc site compared to other European underground laboratories such as Modane does not allow all particles to be stopped in their path through the mountain, making the use of a muon veto necessary.

After the cryostat was shut down in the summer of 2025 and several months of cooling and optimization, data collection began at the end of 2025. Although modest in size compared to large international experiments, CROSS aims to achieve a sensitivity of around 10²⁵ years on the half-life of the double beta decay without neutrinos of molybdenum-100 after one year of operation, meaning that it will be able to test the existence of this process down to levels of rarity corresponding to a decay per nucleus over 10²⁵ years. “It is unlikely that we will observe the phenomenon directly, unless molybdenum-100 proves to be extraordinarily favorable to this type of interaction. In any case, CROSS can provide the best global limit on this isotope,” says Andrea Giuliani. Above all, the experiment plays a strategic role as a technology demonstrator for CUPID, the future large-scale cryogenic detector of the new generation, which is set to take over from the Italian CUORE experiment, which has the same scientific objective, over the next decade, on a larger scale and using a wider variety of isotopes. 

While waiting for CUPID, the CROSS team will provide initial performance analyses of its detectors in the coming weeks, while the first physics results are expected to be unveiled in 2026, opening a new chapter in the quest to understand the intimate nature of neutrinos.