The CROSS experiment: detector construction, background projection, and sensitivity to ${}^{100}$Mo $0\nu 2{\hat{I}}^2$ decay

Abstract

The CROSS experiment to search for neutrinoless double-beta ($0\nu 2{\hat{I}}^2$) decay in ${}^{100}$Mo with the help of an array of scintillating cryogenic calorimeters, containing 4.9 kg of ${}^{100}$Mo, has been ongoing in a low-background setup at the Canfranc underground laboratory (Spain) since mid-November 2025. In this paper, we present the construction of the CROSS detector and the description of Geant4-based Monte Carlo simulations of expected background in the region of interest. The simulations predict the background index in a 100-keV-wide interval centered at the $Q$-value of ${}^{100}$Mo (3034 keV) on the level of 3.2(5) $\times$ 10${}^{-3}$ cnts/keV/kg/yr. Taking into account an 18% deadtime induced by the muon veto cut and a typical 90% duty cycle of the facility, such background level would allow to reach the world-leading sensitivity to ${}^{100}$Mo $0\nu 2{\hat{I}}^2$ decay (lim $T_{1/2}\sim 4\times 10^{24}$ yr) in 1 year of data taking. Considering conservatively a factor 3 (10) worse background index due to unpredictable radioactive contamination of construction materials and/or detector performance, a 2-yr-long operation of the CROSS array would be still compatible with the best (competitive) sensitivity to $0\nu 2{\hat{I}}^2$ decay in ${}^{100}$Mo.

16 pages, 14 figures and 1 table; prepared for submission to EPJC

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