The aim of DELight (Direct search Experiment for light Dark Matter) is to exploit Dark Matter-electron scattering and extend the sensitivity of direct Dark Matter searches with germanium (Ge) crystals by 2 to 3 orders of magnitude in the Dark Matter particle mass, i.e. from the GeV scale down to 1-10 MeV. This MeV-GeV mass region of light Dark Matter, still largely unexplored, is an interesting parameter space predicted by many theoretical models. Even ALP and dark photon Dark Matter in the eV-keV mass scale can be probed with unprecedented sensitivity.
The underlying Dark Matter detection principle consists in measuring ionisation signals of a single or a few electrons resulting from Dark Matter-electron scattering. With an effective band gap of only 3 eV, Ge semiconductors are an excellent probe to access light Dark Matter with masses down to 1 MeV, surpassing significantly liquid noble gases.
The experimental challenge consists of detecting a single electron in a quasi background-free regime in a set of detectors with masses up to the kg-scale, which is significantly larger than what has been achieved recently with tiny silicon crystals of masses of less than 1g. The new approach of DELight is to combine Ge semiconductors as Dark Matter target with Metallic Magnetic Calorimeters (MMCs, developed by our partner group at University Heidelberg) to read out the phonon signal originating from the energy released by free electron-hole pairs drifting to the electrodes. We will apply the Neganov-Luke effect, where charges drifting through a crystal produce additional heat which is proportional to the applied voltage across the detector, to amplify the phonon signal to reach good signal-to-noise ratio for single electron events.
As recently demonstrated with Ge bolometers in the EDELWEISS experiment, an amplification factor of ~35 can be reached by applying a bias of 100 V across a cylindrical Ge monocrystal at an operating temperature of 18mK. The first prototype detector for DELight has already been manufactured and is currently being tested. The detector design consists of a three-fold MMC structure providing three thermal point contacts to the Ge crystal. We developed a new scheme for the installation of a HV vacuum electrode with the possibility of reading out ionisation signals. This first prototype is expected to be fully functional by mid-2018.