Scalable readout for future dark matter searches

The Transition-Edge Sensor (TES) is a very sensitive device formed by a thin film superconductor biased in the middle of its superconducting transition.  Very small changes in temperature - perhaps induced by absorption of free-space radiation, a high-energy particle, or energy deposited through a neutrino or dark matter interaction - induce small electrical signals that can be amplified and digitized.  TESs are broadly used for state-of-the-art measurements for detecting gamma-ray and x-rays, far-infrared and millimeter-wave radiation, and athermal phonons from rare event energy deposits.

A key requirement is that these TES detectors need to be extremely thermally isolated from their environment in order to cool them to their operating temperature of ~0.01 Kelvin, but they also must be electrically connected to circuitry which interacts with conventional electronics at ~300 Kelvin. Many experiments need thousands of these sensors inside a single cryovessel, at which point wiring them all individually becomes very difficult to manage, cool, and fabricate at scale. The solution is to incorporate a cryogenic channelization scheme that drastically cuts down on the number of independent readout units required.

Our group has expertise in developing highly multiplexed readout for TESs sensitive to the cosmic microwave background radiation, including the record largest multiplexing factor for TESs.   We are now looking into broadening the application space for this multiplexing technology. Of particular interest are experiments searching for galactic dark matter via nuclear recoil or electron scattering. As can be seen in the figure below, there is a wide range of unexplored parameter space for masses below the traditional WIMP mass. Interactions from low-mass dark matter particles would deposit much less energy in a target than a WIMP would, requiring very low detection energy thresholds - below those of conventional particle physics detectors based on ionization and photomultipler tubes but within reach of solid-state low-temperature detectors employing TESs. 

Current projects are employing just a few TESs, but in order to cover the full parameter space will need arrays of at least 1,000 sensors. We are therefore working to adapt microwave SQUID multiplexing technology, which has recently become the gold standard for TES arrays measuring high-energy photons and low-energy continuous radiation, to be compatible with sub-eV threshold TES-based athermal phonon detectors to open a path towards these envisioned future kilo-sensor experiments.