Improved dark matter search results from PICO-2L Run 2

New data are reported from a second run of the 2-liter PICO-2L C3F8 bubble chamberwith a total exposure of 129 kg-days at a thermodynamic threshold energyof 3.3 keV. These data show that measures taken to control particulate contamination in the superheatedfluid resulted in the absence of the anomalous background events observed in the first run of this bubble chamber. One single nuclear- recoilevent was observed in the data, consistentboth with the predicted background rate from neutrons and with the observed rate of unambiguous multiple-bubble neutron scattering events. The chamber exhibits the same excellent electron- recoiland alpha decayrejection as was previously reported. These data provide the most stringent direct detection constraints on weakly interacting massive particle( WIMP)-proton spin-dependent scattering to date for WIMPmasses <50 GeV/c2.