A comprehensive characterization of optical superconducting transition-edge sensors for rare event searches

Superconducting Transition-Edge Sensors (TESs) have recently emerged as powerful single-photon detectors due to their high efficiency, low noise, and remarkable sensitivity across a broad spectrum of wavelengths. Within a dark matter experiment focused on searching for dark matter dark photons, we present an unprecedented, comprehensive study of TES background rates and signal characteristics. Achieving an effective dark count rate as low as 10^-4 Hz in the optical to near-infrared wavelength range, we set a new benchmark for optical TESs. Utilizing principal component analysis, we successfully distinguish between photon-like and high-energy events, the latter likely being caused by cosmic rays or natural occuring radioactivity. To assess this hypothesis, we perform coincidence measurements with external detectors to tag cosmic ray events in the TES and conduct additional studies to gauge the impact of natural radioactivity on the TES background. These advancements significantly enhance the applicability of TESs in experiments for rare event searches.