The SIMPLE Phase II dark matter search

Phase II of SIMPLE (Superheated Instrument for Massive Particle Experiments) searched for astroparticle dark matter using superheated liquid C sub(2)ClF sub(5) droplet detectors. Each droplet generally requires an energy deposition with linear energy transfer (LET) [> ~] 150 keV mu m for a liquid-to-gas phase transition, providing an intrinsic rejection against minimum ionizing particles of order 10 super(-10), and reducing the backgrounds to primarily alpha and neutron-induced recoil events. The droplet phase transition generates a millimetric-sized gas bubble that is recorded by acoustic means. We describe the SIMPLE detectors, their acoustic instrumentation, and the characterizations, signal analysis and data selection, which yield a particle-induced, "true nucleation" event detection efficiency of better than 97% at a 95% C.L. The recoil- alpha event discrimination, determined using detectors first irradiated with neutrons and then doped with alpha emitters, provides a recoil identification of better than 99%; it differs from those of COUPP and PICASSO primarily as a result of their different liquids with lower critical LETs. The science measurements, comprising two shielded arrays of fifteen detectors each and a total exposure of 27.77 kgd, are detailed. Removal of the 1.94 kgd Stage 1 installation period data, which had previously been mistakenly included in the data, reduces the science exposure from 20.18 to 18.24 kgd and provides new contour minima of [sigma] sub(p) = 4.3 x 10 super(-3) pb at 35 GeV /c super(2) in the spin-dependent sector of astroparticle dark matter-proton interactions and [sigma] sub(N) = 3.6 x 10 super(-6) pb at 35 GeV /c super(2) in the spin-independent sector. These results are examined with respect to the fluorine spin and halo parameters used in the previous data analysis.