Atoms are made of charged fermions: electrons and quarks. Their behavior is described by the Dirac equation, which also makes a profound prediction: for every particle there exists an antiparticle with the same mass and opposite charge. Neutrinos, however, stand out as a fascinating exception. They are fermions too, yet they carry no electric charge. In the standard picture, they are treated like Dirac particles, much like electrons, but with the “lepton number” playing the role normally occupied by electric charge. There is, however, an intriguing alternative. Neutrinos might instead be Majorana particles: fermions that are completely neutral and identical to their own antiparticles. Confirming this possibility is one of the most important and far-reaching goals in modern particle physics and cosmology.
The only practical avenue to determine whether neutrinos are Majorana particles is to observe neutrinoless double beta decay (0νDBD). This is a hypothetical nuclear process in which an even–even nucleus transforms into a lighter isobar containing two more protons and emits only two electrons: no neutrinos, no other particles. Current experiments have established lower limits on the 0νDBD half-life in the range of 10²⁴ to 10²⁶ years. Yet none of them can reach the extraordinary sensitivity of 10²⁷ to 10²⁸ years, where the probability of a definitive discovery becomes compelling. The CROSS technology aims to bridge exactly this gap, enabling a dramatic leap forward into this unexplored region of sensitivity.
Beyond neutrino physics, the methods pioneered by CROSS promise major advances in radiation detection and in the identification of minute traces of radioactive contaminants. Detectors based on CROSS techniques can measure extremely low neutron fluxes with exceptional spectroscopic precision, opening new possibilities for environmental monitoring, nuclear non-proliferation verification, and homeland security.
CROSS is built around arrays of bolometers whose active materials contain two of the most promising isotopes for 0νDBD searches: ¹⁰⁰Mo and ¹³⁰Te, embedded respectively in crystals of lithium molybdate (Li2MoO₄) and tellurium dioxide (TeO2). Through a series of ambitious innovations, CROSS is developing an approach capable of achieving zero background at exposures of one tonne of isotope per year, an essential condition given the extreme rarity of the process we seek.
The central challenge in 0νDBD searches is the suppression of background events. In bolometric detectors, the most problematic contributions arise from energy depositions near the crystal surfaces. CROSS is developing a technique to distinguish these surface events from those occurring in the bulk, where a true 0νDBD signal would originate. This is accomplished through pulse-shape discrimination enabled by superconducting films deposited on the crystal surfaces. An additional major innovation is a new generation of highly sensitive bolometric light detectors whose response is enhanced by an electric field applied to the light absorber. This allows for powerful rejection of additional backgrounds, including alpha particles and random coincidences of unrelated events. The core ideas of CROSS are being validated in aboveground tests and in a large pilot experiment known as CROSS demonstrator, placed underground in the Canfranc laboratory to be shielded against the cosmic radiation. The entire program is naturally intertwined with CUPID, one of the most promising next-generation bolometric experiments.
