FOR 6006:  DELight: Direct Search Experiment for Light Dark Matter with Superfluid Helium

• Subject Area: Physics
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 561595909

There is compelling evidence from cosmological and astrophysical observations that points to the existence of a new type of matter, the "Dark Matter" (DM). Its nature has yet to be discovered, though. Plausible DM particle candidates include Weakly Interacting Massive Particles (WIMPs) with masses at the GeV-TeV scale. Direct detection experiments search for rare signatures of WIMPs scattering off target nuclei. However, the fact that the simplest WIMP models have already been excluded by such experiments motivates searches for models beyond the traditional WIMP. Very promising are Light Dark Matter (LDM) models with sub-GeV masses, parameter space which experiments have barely been able to probe thus far. The proposed Research Unit (RU) will design, build, and operate the new experiment DELight, with high sensitivity down to sub-100 MeV masses, a milestone in DM-nucleus scattering searches. It will include members from the University of Freiburg, Heidelberg University, and the Karlsruhe Institute of Technology. DELight will be installed locally, first above and then below ground. Very light target nuclei and an ultra-low energy detection threshold are necessary to probe unprecedentedly low DM masses. DELight will be using superfluid helium-4 which is not yet used in an operational DM detector. Helium has no long-lived radioisotopes, is self-cleaning at superfluid temperatures, has a high impedance to external vibration noise, and provides both photon and quasiparticle signal channels valuable for event classification. Background events near the boundaries of the helium target, induced by radioactivity of surrounding materials, can be mitigated by careful material selection, shielding and fiducialization. Furthermore, helium detectors should be rather easily scalable. As helium-4 remains liquid down to zero Kelvin, energy measurements can be performed with ultra-sensitive cryogenic calorimeters using state-of-the-art quantum sensing methods. DELight will employ Magnetic MicroCalorimeters (MMCs), that will be operated below 20 mK and promise very low thresholds of a few eV. By combining their strong expertise on direct DM searches, condensed matter physics, ultra-low temperatures and quantum sensors, the members of the RU will be able to design and build the low-background LDM detector DELight, with a superfluid helium target instrumented with MMCs, and to read, process and analyze its data during a first funding period. With an anticipated energy threshold of 20 eV and an exposure of 1 kg·d acquired in this first phase of the experiment, a scattering cross section of <1e-39 cm^2 can be reached at 200 MeV, improving on current constraints by three orders of magnitude, and LDM masses notably below 100 MeV become accessible for the first time. The RU will have a significant impact on the international DM community, enrich local research, and will foster fruitful collaboration and scientific exchange between the three member institutions.

• DFG Programme: Research Units

Projects

• Coordination Funds (Applicant von Krosigk, Belina )
• Cryogenic setup and superfluid helium cell (Applicant Enss, Christian Walter Dietrich )
• DELight: Data acquisition and computing (Applicant von Krosigk, Belina )
• DELight: Low-background environment (Applicant Schumann, Marc )
• Detector response and analysis (Applicant Valerius, Kathrin )
• MMC-based detection system (Applicant Kempf, Sebastian )
• Site and infrastructure (Applicant Lindemann, Sebastian )

• Spokesperson: Professorin Dr. Belina von Krosigk