Identification of the low-energy excess in dark matter searches with crystal defects

Abstract

An excess of events of unknown origin at low energies below 1 keV has been observed in multiple low-threshold dark matter detectors. Understanding the origin of these events is of utmost importance, as this unidentified event rate currently overwhelms any potential new physics signal. Depending on the target material, nuclear recoil events at these energies may cause lattice defects, in which case a part of the true recoil energy is stored in the defect and not observed in the phonon detector. If the threshold for defect creation is sharp, this effect leads to a prominent feature in the observed recoil spectrum. Electronic recoils at low energies do not create defects and therefore the feature in the observed spectrum is not expected in that case. We propose to use the sharp defect creation threshold of diamond to test if the low-energy events are due to nuclear recoils. Based on simulated data we expect the nuclear recoil peak in the observed spectrum to be visible in diamond with a data set of $\ensuremath{\sim}700$ events, potentially achievable with $\ensuremath{\sim}0.1$ gram days of exposure.