Low-temperature optical photon detectors for the CUPID experiment.

CUPID is a proposed next-generation experiment that will search for neutrinoless double-β (0νββ) decay in 100Mo using ~1600 Li2100MoO4 scintillating crystals operated as low-temperature calorimeters close to ∼10mK. It will leverage the crystal's energy loss mechanism to tag particle type by simultaneously measuring thermal and scintillation signals. We will use an auxiliary low-temperature calorimeter to detect light with high photon collection efficiency. The light detectors must have a very low energy threshold ??(100eV) and good timing resolution < 0.17 μs to tag α background and 2νββ pile-up events in the region of interest. The light detectors are crucial to reach the CUPID background goal of <1E-4 counts/(keV.kg.yr) for its baseline design. In this talk, I will briefly discuss the baseline design of the CUPID light detectors and the R&D status of a future upgrade with transition-edge sensor (TES) based light detectors which may further help suppress the pile-up background. This work will present the detector's energy and timing resolution performance and its response to optical photons and x-rays.