First dark matter search results from a 4-kg CF$_3$I bubble chamber
  operated in a deep underground site
  New data are reported from the operation of a 4.0 kg CF$_{3}$I bubble chamber
in the 6800-foot-deep SNOLAB underground laboratory. The effectiveness of
ultrasound analysis in discriminating alpha-decay background events from single
nuclear recoils has been confirmed, with a lower bound of $>$99.3% rejection of
alpha-decay events. Twenty single nuclear recoil event candidates and three
multiple bubble events were observed during a total exposure of 553 kg-days
distributed over three different bubble nucleation thresholds. The effective
exposure for single bubble recoil-like events was 437.4 kg-days. A neutron
background internal to the apparatus, of known origin, is estimated to account
for five single nuclear recoil events and is consistent with the observed rate
of multiple bubble events. This observation provides world best direct
detection constraints on WIMP-proton spin-dependent scattering for WIMP masses
$>$20 GeV/c$^{2}$ and demonstrates significant sensitivity for spin-independent
interactions.
