Final results are reported from operation of the PICO-60 ${\mathrm{C}}_3{\mathrm{F}}_8$ dark matter detector, a bubble chamber filled with 52 kg of ${\mathrm{C}}_3{\mathrm{F}}_8$ 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 weakly interacting massive particle (WIMP)-proton spin-dependent cross section at $3.2\times 10^{-41}{\mathrm{cm}}^2$ for a 25 GeV WIMP, improving on previous PICO results for 3–5 GeV WIMPs by an order of magnitude.

• Received 6 March 2019

DOI:https://doi.org/10.1103/PhysRevD.100.022001