Final results are reported from operation of the PICO-60 C _ { 3 } F _ { 8 } dark matter detector , a bubble chamber filled with \SI 52 \kg of C _ { 3 } F _ { 8 } located in the SNOLAB underground laboratory .
The chamber was operated at thermodynamic thresholds as low as \SI 1.2 \keV without loss of stability .
A new blind 1404-kg-day exposure at \SI 2.45 \keV threshold was acquired with approximately the same expected total background rate as the previous 1167-kg-day exposure at \SI 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 \SI 25 \GeV WIMP , and improve on previous PICO results for 3– \SI 5 \GeV WIMPs by an order of magnitude .