We present constraints on the existence of weakly interacting massive particles ( WIMPs ) from a 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 eV _ { 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 _ { 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 .