HeRALD - a sub-GeV dark matter detector based on superfluid He-4

As part of the TESSERACT project searching for sub-GeV dark matter, HeRALD experiment utilizes superfluid helium-4 as target material with transition edge sensors (TES) readout. Superfluid He-4 has multiple advantages for low-mass dark matter search, including a good kinematic match for better energy/momentum transfer, good purity with no radioactive isotopes and convenience of impurity removal with cold trap, multiple readout channels for potential particle identification, liquid phase down to absolute zero temperature for a monolithic detector with TES readout. Progress with recent detector concept demonstration will be discussed, including the deposition of Cs thin film to block the creeping superfluid He-4 film, and observation of delayed signal from quantum evaporation of He-4 atoms from the phonon/roton quasiparticles generated in energy depositions. Quantum evaporation signal is the main feature of this detector concept to achieve low energy threshold, along with the improvement of the energy resolution of the calorimeters with TES readout.