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Cryogenic calorimetric experiments to search for neutrinoless double-beta decay (0νββ) are highly competitive, scalable, and versatile in isotope choice. The largest planned detector array, CUPID, consists of about 1500 individual Li₂¹⁰⁰MoO₄ detector modules, with further scaling envisioned for a follow-up experiment (CUPID-1T). In this article, we present a novel detector concept targeting this second stage, using a low-impedance TES-based readout for the Li₂MoO₄ absorber. This design is easily mass-produced and supports multiplexed readout. We describe the detector design and results from a first prototype operated at the NEXUS shallow underground facility at Fermilab. The detector is a 2-cm-side cube with a mass of 21 g, strongly thermally coupled to its readout chip, allowing rise-times of approximately 0.5 ms. This is more than an order of magnitude faster than current NTD-based detectors and is expected to effectively mitigate backgrounds caused by pile-up of two independent two-neutrino decay events occurring close in time. With a baseline resolution of 1.95 keV (FWHM), these performance parameters extrapolate to a background index from pile-up as low as 5 × 10⁻⁶ counts/keV/kg/year in CUPID-sized crystals. The detector was calibrated up to the MeV region, demonstrating sufficient dynamic range for 0νββ searches. In combination with a SuperCDMS HVeV detector, this setup also enabled a precision measurement of the scintillation time constants of Li₂MoO₄, revealing a primary component with a fast ~20 μs time scale. 

Abstract The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.