A Transition Edge Sensor Operated in Coincidence with a High Sensitivity Athermal Phonon Sensor for Photon Coupled Rare Event Searches

Experimental searches for axions or dark photons that couple to the standard model photon require photosensors with low noise, broadband sensitivity, and near zero backgrounds. Here, we introduce an experimental architecture, in which a small photon sensor, in our case a Transition Edge Sensor (TES) with a photon energy resolution $σ_γ= 368.4 \pm 0.4$ meV, is colocated on the same substrate as a large high sensitivity athermal phonon sensor (APS) with a phonon energy resolution $σ_\mathrm{phonon} = 701 \pm 2$ meV. We show that single 3.061 eV photons absorbed in the photon-sensing TES deposit $\sim$35\% of their energy in the electronic system of the TES, while $\sim$26\% of the photon energy leaks out of the photon-sensing TES during the downconversion process and becomes absorbed by the APS. Backgrounds, which we associate with the broadly observed ``low energy excess'' (LEE), are observed to be largely coupled to either the TES (``singles'' LEE), or phonon system, (``shared'' LEE). At high energies, these backgrounds can be efficiently discriminated from TES photon absorption events, while at low energies, their misidentification as photon events is well modeled. With significant sensitivity improvements to both the TES and APS, this coincidence technique could be used to suppress backgrounds in bosonic dark matter searches down to energies near the superconducting bandgap of the sensor.