Phonon-Mediated KIDs as Light Detectors for Rare Event Search: The CALDER Project

Background suppression plays a crucial role in particle physics experiments searching for rare events, such as neutrinoless double beta decay and dark matter interactions. Bolometers, that are among the most competitive devices in this field, would largely benefit from the development of ultrasensitive light detectors, as the combined readout of the bolometric and light signals enables the particle identification. The CALDER collaboration is developing superconducting light detectors that will match the requirements of next generation experiments: noise lower than 20 eV, large active area ($>$20 cm $^2$), wide temperature range of operation, high radiopurity, and ease in fabricating hundreds of channels. For this purpose, we are exploiting the excellent energy resolution and the natural multiplexed readout provided by kinetic inductance detectors (KIDs). KIDs have already demonstrated their potentiality as direct detectors of photons for different astrophysical applications. The aim of our project is to apply this technology in particle physics, using indirect detection. These devices can be operated in a phonon-mediated approach, in which KIDs are coupled to a large insulating substrates in order to increase the active surface from a few mm$^2$ to 25 cm $^2$. We have already demonstrated the feasibility of a phonon-mediated KIDs-based light detectors, using aluminium sensors. These device reached a baseline sensitivity of around 80 eV with an overall efficiency of about 20%. Currently, we are testing new materials (e.g., Ti-Al and nonstoichiometric TiN) to enhance the sensitivity and reach the goal of our project. We present our results and the physical interpretation of the device behavior. Finally, we also discuss the impact of this project on the most advanced bolometric experiments searching for neutrinoless double beta decay and dark matter.