Orateur
Description
The search for neutrinoless double beta decay ($0\nu\beta\beta$) is one of the most important challenges in contemporary neutrino physics. The observation of this hypothetical process would demonstrate the violation of lepton number conservation and establish the Majorana nature of neutrinos, providing direct evidence for physics beyond the Standard Model. The sensitivity of next-generation experiments is largely determined by their ability to achieve excellent energy resolution while reducing radioactive backgrounds to unprecedented levels.
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The Cryogenic Underground Observatory for Rare Events (CUORE) has demonstrated the maturity of large-scale bolometric detectors. Building on this experience, the next-generation experiment CUPID (CUORE Upgrade with Particle IDentification) aims to explore the inverted neutrino mass ordering region by operating a tonne-scale array of enriched cryogenic calorimeters with active particle identification. To achieve its physics goals, CUPID targets an ultra-low background index corresponding to a nearly background-free experiment over its expected exposure.
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The CROSS demonstrator, installed at the Canfranc Underground Laboratory in Spain and taking data since October 2025, is a standalone experiment while also serving as a large-scale technological demonstrator for CUPID. Its primary objective is to validate the detector technologies, background-rejection strategies, and cryogenic light-detection techniques required for future tonne-scale bolometric experiments. In particular, CROSS provides the first large-scale validation of Germanium and Silicon based cryogenic light detectors coupled to scintillating Li$_2$MoO$_4$ absorbers. The efficient detection of the scintillation light emitted by the crystals is essential for discriminating between $\alpha$ and $\beta/\gamma$ interactions, one of the key requirements for achieving the ultra-low background levels targeted by CUPID. Demonstrating stable operation, high signal-to-noise performance, and reliable particle identification with Silicon light detectors is therefore a crucial milestone toward their deployment in future large-mass experiments.
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The CROSS demonstrator represents a crucial technological milestone on the path toward CUPID, providing a full-scale validation of detector concepts and background-rejection techniques required for tonne-scale cryogenic experiments. By combining excellent energy resolution, active particle identification and Neganov-Trofimov-Luke amplified light detection, CROSS aims to demonstrate the background suppression necessary for the next generation of neutrinoless double beta decay searches. In particular, CROSS will validate Silicon-based cryogenic light detectors for the detection of scintillation light from Li$_2$MoO$_4$ crystals. Their lower cost and greater scalability, while maintaining excellent performance, make them especially attractive for future large-scale experiments such as CUPID. The experiment will therefore provide essential input for CUPID's goal of reaching a background index of the order of $10^{-4}$ counts/(keV,kg,yr), enabling a nearly background-free exploration of the inverted neutrino mass hierarchy region and potentially setting the most stringent limits on the $0\nu\beta\beta$ half-life of $^{100}$Mo.