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 .