Dark Matter Search Results from the PICO-2L C$_3$F$_8$ Bubble Chamber
  New data are reported from the operation of a 2-liter C$_3$F$_8$ bubble
chamber in the 2100 meter deep SNOLAB underground laboratory, with a total
exposure of 211.5 kg-days at four different recoil energy thresholds ranging
from 3.2 keV to 8.1 keV. These data show that C3F8 provides excellent electron
recoil and alpha rejection capabilities at very low thresholds, including the
first observation of a dependence of acoustic signal on alpha energy. Twelve
single nuclear recoil event candidates were observed during the run. The
candidate events exhibit timing characteristics that are not consistent with
the hypothesis of a uniform time distribution, and no evidence for a dark
matter signal is claimed. These data provide the most sensitive direct
detection constraints on WIMP-proton spin-dependent scattering to date, with
significant sensitivity at low WIMP masses for spin-independent WIMP-nucleon
scattering.
