Dark matter electron interactions in detector materials

Abstract: Dark Matter (DM) makes up 85% of the matter content of the universe, and its gravitational effects are seen on scales ranging from that of cosmology to that of galactic astrophysics. The nature of DM is, however, unknown. Studying DM in the lab is key to understanding its nature. For decades, experiments have been attempting to do this through searches for DM induced nuclear recoils. These have not been found, and a possible reason for this is that the hypothetical DM particle is too light to induce nuclear recoils. Therefore, in the last decade experiments have been built to study DM through electron recoils instead. As the electron is 4 orders of magnitude lighter than the nucleus, electron recoils can be induced by DM down to 4 orders of magnitude lighter than the lightest DM particle probeable with nuclear recoils. In order to understand current and upcoming results from experiments searching for DM induced electron recoils, a theoretical understanding of DM electron scatterings in detector materials is needed. This requires input both from dark matter and material physics, and so far DM electron interactions have only been studied within the dark photon model. In the dark photon model, the DM-electron scattering takes a relatively simple form, and the material responds to the scattering through a single "response function".  Relaxing the assumption of the dark photon model, and instead applying Non-Relativistic Effective Theory approach, we calculate the expected detector signature for a wide range of DM models in Silicon and Germanium. In the papers of this thesis, we find that in contrast to the single response function produced by the dark photon model, the material can respond with 7 different response functions. These novel response functions we show to generically arise in a wide range of DM-electron interactions. As such, the papers of this thesis vastly extends the forms of DM-electron interactions that can be studied in Silicon and Germanium based experiments. These interactions made studyable by the works underlying this thesis are not fringe cases, but generically arise in a wide range of models. To illustrate this we consider a range of simplified models with a DM particle with spin 0, spin 1/2 and spin 1.