The SIMPLE Phase II dark matter search

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Abstract

Phase II of SIMPLE (Superheated Instrument for Massive ParticLe Experiments) searched for astroparticle dark matter using superheated liquid ${\mathrm{C}}_{2}{\mathrm{ClF}}_{5}$ droplet detectors. Each droplet generally requires an energy deposition with linear energy transfer (LET) $\ensuremath{\gtrsim}150\text{ }\text{ }\mathrm{keV}/\ensuremath{\mu}\mathrm{m}$ for a liquid-to-gas phase transition, providing an intrinsic rejection against minimum ionizing particles of order ${10}^{\ensuremath{-}10}$, and reducing the backgrounds to primarily $\ensuremath{\alpha}$ 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-$\ensuremath{\alpha}$ 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 ${\ensuremath{\sigma}}_{p}=4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{pb}$ at $35\text{ }\text{ }\mathrm{GeV}/{\mathrm{c}}^{2}$ in the spin-dependent sector of astroparticle dark matter--proton interactions and ${\ensuremath{\sigma}}_{N}=3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\text{ }\text{ }\mathrm{pb}$ at $35\text{ }\text{ }\mathrm{GeV}/{\mathrm{c}}^{2}$ in the spin-independent sector. These results are examined with respect to the fluorine spin and halo parameters used in the previous data analysis.

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