Simulation and measurement of Black Body Radiation background in a Transition Edge Sensor

The Any Light Particle Search II (ALPS II) experiment at DESY, Hamburg, is a Light-Shining-through-a-Wall (LSW) experiment aiming to probe the existence of axions and axion-like particles (ALPs), which are candidates for dark matter. Data collection in ALPS II is underway utilizing a heterodyne-based detection scheme. A complementary run for confirmation or as an alternative method is planned using single photon detection, requiring a sensor capable of measuring low-energy photons ($1064\,\mathrm{nm}$, $1.165\,\mathrm{eV}$) with high efficiency (higher than $50\,\%$) and a low background rate (below $7.7\cdot10^{-6}\,\mathrm{cps}$). To meet these requirements, we are investigating a tungsten Transition Edge Sensor (TES) provided by NIST, which operates in its superconducting transition region at millikelvin temperatures. This sensor exploits the drastic change in resistance caused by the absorption of a single photon. We find that the background observed in the setup with a fiber-coupled TES is consistent with Black Body Radiation (BBR) as the primary background contributor. A framework was developed to simulate BBR propagation to the TES under realistic conditions. The framework not only allows the exploration of background reduction strategies, such as improving the TES energy resolution, but also reproduces, within uncertainties, the spectral distribution of the observed background. These simulations have been validated with experimental data, in agreement with the modeled background distribution, and show that the improved energy resolution reduces the background rate in the $1064\,\mathrm{nm}$ signal region by one order of magnitude, to approximately $10^{-4}\,\mathrm{cps}$. However, this rate must be reduced further to meet the ALPS II requirements.