Dark Matter Search Results from the Complete Exposure of the PICO-60
  C$_3$F$_8$ Bubble Chamber
  Final results are reported from operation of the PICO-60 C$_3$F$_8$ dark
matter detector, a bubble chamber filled with 52 kg of C$_3$F$_8$ located in
the SNOLAB underground laboratory. The chamber was operated at thermodynamic
thresholds as low as 1.2 keV without loss of stability. A new blind 1404-kg-day
exposure at 2.45 keV threshold was acquired with approximately the same
expected total background rate as the previous 1167-kg-day exposure at 3.3 keV.
This increased exposure is enabled in part by a new optical tracking analysis
to better identify events near detector walls, permitting a larger fiducial
volume. These results set the most stringent direct-detection constraint to
date on the WIMP-proton spin-dependent cross section at 2.5 $\times$ 10$^{-41}$
cm$^2$ for a 25 GeV WIMP, and improve on previous PICO results for 3-5 GeV
WIMPs by an order of magnitude.
