HeRALD: R&D Toward a Sub-GeV Dark Matter Search Using Superfluid Helium

Part of the broader TESSERACT experiment, HeRALD seeks to use a target of superfluid helium and transition edge sensors (TESs) with sub-eV energy resolution to search for sub-GeV mass dark matter. Helium offers many favorable characteristics for a dark matter target, including ease of chemical and radiopurity, kinematic matching for optimal momentum transfer in nuclear recoils, scalability, and multiple signal channels for discrimination between electron and nuclear recoils. So far, we have successfully demonstrated rollin film blocking using thin films of cesium, and observed both evaporation and scintillation signals from electron recoils in the helium. We have also developed calibration techniques using inorganic scintillators. We are actively developing a testbed featuring multiple independent TESs in an effort to mitigate the effects of heretofore unexplained low-energy excess (LEE) events. In the future, HeRALD will occupy space at Laboratoire Souterrain de Modane (LSM) for an extended dark matter search.