Improved dark matter search results from PICO-2L Run 2
  New data are reported from a second run of the 2-liter PICO-2L C$_3$F$_8$
bubble chamber with a total exposure of 129$\,$kg-days at a thermodynamic
threshold energy of 3.3$\,$keV. These data show that measures taken to control
particulate contamination in the superheated fluid resulted in the absence of
the anomalous background events observed in the first run of this bubble
chamber. One single nuclear-recoil event was observed in the data, consistent
both 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-recoil and alpha decay rejection 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 WIMP masses $<$ 50$\,$GeV/c$^2$.
