  We present constraints on the existence of weakly interacting massive
particles (WIMPs) from an 11 kg-day 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 $>$200
eV$_{\rm 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 eV$_{\rm ee}$. 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 $3\times 10^{-41}$ cm$^2$
for WIMPs with masses $m_{\chi}$ from 7 to 10 GeV$c^{-2}$ . These results are
the strongest constraints from a silicon target on the existence of WIMPs with
$m_{\chi}$$<$9 GeV$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.
