DEAP-3600 is a single-phase liquid argon (LAr) direct-detection dark matter experiment, operating 2 km underground at SNOLAB (Sudbury, Canada). The detector consists of 3279 kg of LAr contained in a spherical acrylic vessel. This paper reports on the analysis of a 758 tonne · day exposure taken over a period of 231 live-days during the first year of operation. No candidate signal events are observed in the WIMP-search region of interest, which results in the leading limit on the WIMP-nucleon spin-independent cross section on a LAr target of 3.9 × 10 −45 cm 2 (1.5 × 10 −44 cm 2 ) for a 100 GeV=c 2 (1 TeV=c 2 ) WIMP mass at 90% C.L. In addition to a detailed background model, this analysis demonstrates the best pulseshape discrimination in LAr at threshold, employs a Bayesian photoelectron-counting technique to improve the energy resolution and discrimination efficiency, and utilizes two position reconstruction algorithms based on the charge and photon detection time distributions observed in each photomultiplier tube.
The Dark matter Experiment using Argon Pulse-shape discrimination (DEAP) has been designed for a direct detection search for particle dark matter using a single-phase liquid argon target. The projected cross section sensitivity for DEAP-3600 to the spin-independent scattering of Weakly Interacting Massive Particles (WIMPs) on nucleons is 10 −46 cm 2 for a 100 GeV/c 2 WIMP mass with a fiducial exposure of 3 tonne-years. This paper describes the physical properties and construction of the DEAP-3600 detector.
This Letter reports the first results of a direct dark matter search with the DEAP-3600 single-phase liquid argon (LAr) detector. The experiment was performed 2 km underground at SNOLAB (Sudbury, Canada) utilizing a large target mass, with the LAr target contained in a spherical acrylic vessel of 3600 kg capacity. The LAr is viewed by an array of PMTs, which would register scintillation light produced by rare nuclear recoil signals induced by dark matter particle scattering. An analysis of 4.44 live days (fiducial exposure of 9.87 ton day) of data taken during the initial filling phase demonstrates the best electronic recoil rejection using pulse-shape discrimination in argon, with leakage <1.2×10^{-7} (90% C.L.) between 15 and 31 keV_{ee}. No candidate signal events are observed, which results in the leading limit on weakly interacting massive particle (WIMP)-nucleon spin-independent cross section on argon, <1.2×10^{-44} cm^{2} for a 100 GeV/c^{2} WIMP mass (90% C.L.).
Lepton family number violation is tested by searching for µ + → e + X 0 decays among the 5.8×10 8 positive muon decay events analyzed by the TWIST collaboration. Limits are set on the production of both massless and massive X 0 bosons. The large angular acceptance of this experiment allows limits to be placed on anisotropic µ + → e + X 0 decays, which can arise from interactions violating both lepton flavor and parity conservation. Branching ratio limits of order 10 −5 are obtained for bosons with masses of 13 -80 MeV/c 2 and with different decay asymmetries. For bosons with masses less than 13 MeV/c 2 the asymmetry dependence is much stronger and the 90% limit on the branching ratio varies up to 5.8 × 10 −5 . This is the first study that explicitly evaluates the limits for anisotropic two body muon decays.
The TWIST Collaboration has completed its measurement of the three muon decay parameters , , and P . This paper describes our determination of , which governs the shape of the overall momentum spectrum, and , which controls the momentum dependence of the parity-violating decay asymmetry. The results are ¼ 0:749 77 AE 0:000 12ðstatÞ AE 0:000 23ðsystÞ and ¼ 0:750 49 AE 0:000 21ðstatÞ AE 0:000 27ðsystÞ. These are consistent with the value of 3=4 given for both parameters in the standard model, and each is over a factor of 10 more precise than the measurements published prior to TWIST. Our final results on , , and P have been incorporated into a new global analysis of all available muon decay data, resulting in improved modelindependent constraints on the possible weak interactions of right-handed particles.
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