Dark Matter Search Results from the PICO-60 C$_3$F$_8$ Bubble Chamber
  New results are reported from the operation of the PICO-60 dark matter
detector, a bubble chamber filled with 52 kg of C$_3$F$_8$ located in the
SNOLAB underground laboratory. As in previous PICO bubble chambers, PICO-60
C$_3$F$_8$ exhibits excellent electron recoil and alpha decay rejection, and
the observed multiple-scattering neutron rate indicates a single-scatter
neutron background of less than 1 event per month. A blind analysis of an
efficiency-corrected 1167-kg-day exposure at a 3.3-keV thermodynamic threshold
reveals no single-scattering nuclear recoil candidates, consistent with the
predicted background. These results set the most stringent direct-detection
constraint to date on the WIMP-proton spin-dependent cross section at 3.4
$\times$ 10$^{-41}$ cm$^2$ for a 30-GeV$\thinspace$c$^{-2}$ WIMP, more than one
order of magnitude improvement from previous PICO results.
