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 $>200{\mathrm{eV}}_{\mathrm{ee}}$ in the DAMIC CCDs are consistent with backgrounds from natural radioactivity. An excess of ionization events is observed above the analysis threshold of $50{\mathrm{eV}}_{\mathrm{ee}}$. While the origin of this low-energy excess requires further investigation, our data exclude spin-independent WIMP-nucleon scattering cross sections ${\sigma }_{\chi -n}$ as low as $3\times {10}^{-41}{\mathrm{cm}}^2$ for WIMPs with masses $m_{\chi }$ from 7 to $10\mathrm{GeV}{c}^{-2}$. These results are the strongest constraints from a silicon target on the existence of WIMPs with $m_{\chi }<9\mathrm{GeV}{c}^{-2}$ and are directly relevant to any dark matter interpretation of the excess of nuclear-recoil events observed by the CDMS silicon experiment in 2013.

• Received 2 August 2020
• Accepted 4 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.241803