Results on Low-Mass Weakly Interacting Massive Particles from an 11 kg d Target Exposure of DAMIC at SNOLAB

Abstract

We present constraints on the existence of weakly interacting massive particles (WIMPs) from an 11 kg d target exposure of the DAMIC experiment at the SNOLAB underground laboratory. The observed energy spectrum and spatial distribution of ionization events with electron-equivalent energies $>200\text{ }\text{ }{\mathrm{eV}}_{\mathrm{ee}}$ in the DAMIC CCDs are consistent with backgrounds from natural radioactivity. An excess of ionization events is observed above the analysis threshold of $50\text{ }{\mathrm{eV}}_{\mathrm{ee}}$. While the origin of this low-energy excess requires further investigation, our data exclude spin-independent WIMP-nucleon scattering cross sections ${\ensuremath{\sigma}}_{\ensuremath{\chi}\ensuremath{-}n}$ as low as $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}41}\text{ }\text{ }{\mathrm{cm}}^{2}$ for WIMPs with masses ${m}_{\ensuremath{\chi}}$ from 7 to $10\text{ }\mathrm{GeV}\text{ }{c}^{\ensuremath{-}2}$. These results are the strongest constraints from a silicon target on the existence of WIMPs with ${m}_{\ensuremath{\chi}}<9\text{ }\mathrm{GeV}\text{ }{c}^{\ensuremath{-}2}$ and are directly relevant to any dark matter interpretation of the excess of nuclear-recoil events observed by the CDMS silicon experiment in 2013.