Dark Matter Search Results from the PICO-60 CF$_3$I Bubble Chamber
  New data are reported from the operation of the PICO-60 dark matter detector,
a bubble chamber filled with 36.8 kg of CF$_3$I and located in the SNOLAB
underground laboratory. PICO-60 is the largest bubble chamber to search for
dark matter to date. With an analyzed exposure of 92.8 livedays, PICO-60
exhibits the same excellent background rejection observed in smaller bubble
chambers. Alpha decays in PICO-60 exhibit frequency-dependent acoustic
calorimetry, similar but not identical to that reported recently in a
C$_3$F$_8$ bubble chamber. PICO-60 also observes a large population of unknown
background events, exhibiting acoustic, spatial, and timing behaviors
inconsistent with those expected from a dark matter signal. These behaviors
allow for analysis cuts to remove all background events while retaining
$48.2\%$ of the exposure. Stringent limits on weakly interacting massive
particles interacting via spin-dependent proton and spin-independent processes
are set, and most interpretations of the DAMA/LIBRA modulation signal as dark
matter interacting with iodine nuclei are ruled out.
