First background-free limit from a directional dark matter experiment:
  results from a fully fiducialised DRIFT detector
  The addition of O2 to gas mixtures in time projection chambers containing CS2
has recently been shown to produce multiple negative ions that travel at
slightly different velocities. This allows a measurement of the absolute
position of ionising events in the z (drift) direction. In this work, we apply
the z-fiducialisation technique to a directional dark matter search. In
particular, we present results from a 46.3 live-day source-free exposure of the
DRIFT-IId detector run in this completely new mode. With full-volume
fiducialisation, we have achieved the first background-free operation of a
directional detector. The resulting exclusion curve for spin-dependent
WIMP-proton interactions reaches 1.1 pb at 100 GeV/c2, a factor of 2 better
than our previous work. We describe the automated analysis used here, and argue
that detector upgrades, implemented after the acquisition of these data, will
bring an additional factor of >3 improvement in the near future.
