Origin of photonlike dark-count events in optical TESs

Determine the physical origin and generation mechanism of single-photon–like dark-count events observed by the Ti/Au optical transition-edge sensor (TES E) operated inside an adiabatic demagnetization refrigerator with no optical fiber or external light source, which persist at a rate of 3.6×10^-4 Hz in the 0.8–3.2 eV energy range (6×10^-5 Hz near 1.5±0.2 eV) and remain unchanged when external gamma-ray sources (232Th and 22Na) are present. Clarify whether these photonlike events arise from stray optical photons, unidentified instrumental effects, or other physical processes, to enable accurate background modeling for dielectric haloscope dark-photon searches.

Background

The paper characterizes backgrounds in optical superconducting transition-edge sensors (TESs), distinguishing photonlike, high-energy, and electrical-noise events, and develops automated classification tools. Through dedicated runs with radioactive sources (232Th and 22Na), a cosmic-ray coincidence setup, and detailed GEANT4 simulations, the authors attribute high-energy events to cosmic rays and environmental gamma rays interacting with the TES substrate.

In contrast, the rate of photonlike events remains constant across configurations and sources, suggesting that gamma rays do not contribute. The simulations do not include photonlike rates due to their unknown origin. Identifying the source of these photonlike dark counts is crucial for rare-event searches, especially dielectric haloscopes targeting dark-photon dark matter, which expect genuine single-photon–like signals.