Final Analysis and Results of the Phase II SIMPLE Dark Matter Search

Abstract

We report the final results of the Phase II SIMPLE measurements, comprising two run stages of 15 superheated droplet detectors each, the second stage including an improved neutron shielding. The analyses includes a refined signal analysis, and revised nucleation efficiency based on reanalysis of previously-reported monochromatic neutron irradiations. The combined results yield a contour minimum of σp = 5.7 × 10 −3 pb at 35 GeV/c 2 in the spin-dependent sector of WIMP-proton interactions, the most restrictive to date for MW ≤ 60 GeV/c 2 from a direct search experiment and overlapping for the first time results previously obtained only indirectly. In the spin-independent sector, a minimum of 4.7 × 10 −6 pb at 35 GeV/c 2 is achieved, with the exclusion contour challenging a significant part of the light mass WIMP region of current interest.The search for weakly interacting massive particle (WIMP) dark matter remains at the forefront of modern physics activity. Estimated to comprise ∼ 23% of the Universe mass, it is the role of direct detection efforts to elaborate its nature, and whether its interaction with nucleons is spin-independent (SI) or spin-dependent (SD). SIMPLE (Superheated Instrument for Massive ParticLe Experiments) [1] is a direct search activity using superheated liquid detectors, and one of only a few in the international panorama with sensitivity to the WIMPproton sector of the SD phase space. It is operated at the 1500 mwe level of the Low Noise Underground Laboratory (LSBB) in southern France.In [1], we reported the first results of a two stage Phase II measurement, comprising a 14.1 kgd Stage 1 exposure of 15 superheated droplet detectors (SDDs) [2-4] with a total active mass of 0.208 kg. We here provide the results of the full Phase II measurement, including a 13.67 kgd Stage 2 exposure of a second 15 SDD set, together with improved neutron shielding and a refined analysis of the individual detector run signals, sensitivities, and nucleation efficiency.A SDD consists of a dispersion of superheated liquid droplets homogeneously distributed within a gel matrix, which may undergo a transition to the gas phase upon energy deposition by incident radiation. Two conditions are required for the nucleation of the gas phase of the superheated droplets [5]: (i) the energy deposited must be greater than a thermodynamic minimum, and (ii) this * criodets@cii.fc.ul.pt energy must be deposited within a thermodynamicallydefined minimum distance (Λr c ) inside the droplet, where Λ is the nucleation parameter and r c = the thermodynamic critical bubble radius. Adjustment of the two conditions results in the necessity of depositions of order ≥ 150 keV/µm for a bubble nucleation, rendering the SDD effectively insensitive to the majority of traditional detector backgrounds (including electrons, γ's and cosmic muons) which complicate more conventional dark matter search detectors, leaving only α-and neutron-induced events.The 15 Stage 2 SDDs were fabricated as described in [1], each containing between 11-19 g of C 2 ...

Overcoming velocity suppression in dark-matter direct-detection experiments

“…Several direct-detection experiments already provide comparable, stringent limits on spin-dependent scattering [23,30,31]. Moreover, the next generation of these experiments, including COUPP-60 and PICO-250L, are projected to significantly extend the reach of these experiments in the near future [32].…”

Cornering light neutralino dark matter at the LHC

“…Bounds on the σ SD from direct detection experiments such as PICASSO [57], COUPP [58], SIMPLE [59], KIMS [60] IceCube [61], XENON100 [62] (see also [63]) and the Baksan Neutrino Observatory [64] are not much constraining yet for light neutralino DM. Nevertheless, LHC searches for events with a monojet plus missing transverse momentum [65,66] can be translated into bounds on neutralino-nucleon scattering crosssections [67,68,69,70].…”

Exothermic double-disk dark matter

“…Also, as the modulation amplitude is suppressed for any exothermic model of DM, limits from modulation studies will be further weakened. For the sake of thoroughness we also consider limits from the SIMPLE experiment [42] and the analysis of CRESST-II commissioning run data which includes oxygen recoils to push sensitivity to lower DM masses [43]. As well as the 90% best-fit region for the CDMS-Si events [15], we also consider 90% best-fit regions for the CRESST-II excess of events [23], however it should be noted that this excess, or at least a significant portion of it, may be accounted for with additional background sources [44].…”

“…We take the cut acceptance and nucleation efficiency from [42]. We use the Feldman-Cousins method [78] to determine a 90% upper limit of 2.38 signal events based on an expected background of 2.2 events and the observation of one event above the 8 keV threshold.…”

“…• SIMPLE: We use the Stage 2 results [42], of the C 2 ClF 5 detector with 6.71 kg days exposure. We take the cut acceptance and nucleation efficiency from [42].…”