Understanding Ionization Efficiency from Sub-keV Nuclear Recoil Events in Direct Detection Dark Matter Experiments.

Direct detection dark matter experiments are increasingly interested in the low-mass dark matter parameter space.  But sensitivity to low mass demands lower thresholds, and with thresholds as low as 54eVee for CDMSlite, there are few and sometimes no nearby measurements of the ionization efficiency of nuclear recoils. The dark matter community widely describes the ionization efficiency from a nuclear recoil with the Lindhard model, but experimental disagreements below 1 keVnr suggest that some of its assumptions, such as treating the atomic binding energy as negligible, may break down. This uncertainty in model predictions and detector response introduces significant error into a low mass parameter space as we do not know if there is a minimum energy required to produce an electron-hole pair from a nuclear recoil. To further understand the minimum recoil energy required to create an e/h pair, more data below 1keVnr is needed. Here, I discuss potential experiments to study low-energy nuclear recoil. In particular I look at the sensitivity of detectors with single e/h resolution in order to verify a modified Lindhard model and to test for directional dependence of the detector response.