D. Yu. Akimov

WIMP-nucleon cross-section results from the second science run of ZEPLIN-III

Abstract We report experimental upper limits on WIMP-nucleon elastic scattering cross sections from the second science run of ZEPLIN-III at the Boulby Underground Laboratory. A raw fiducial exposure of 1344 kg⋅days was accrued over 319 days of continuous operation between June 2010 and May 2011. A total of eight events was observed in the signal acceptance region in the nuclear recoil energy range 7–29 keV, which is compatible with background expectations. This allows the exclusion of the scalar cross-section above 4.8 × 10 − 8 pb near 50 GeV / c 2 WIMP mass with 90% confidence. Combined with data from the first run, this result improves to 3.9 × 10 − 8 pb . The corresponding WIMP-neutron spin-dependent cross-section limit is 8.0 × 10 − 3 pb . The ZEPLIN programme reaches thus its conclusion at Boulby, having deployed and exploited successfully three liquid xenon experiments of increasing reach.

The ZEPLIN-III dark matter detector: Instrument design, manufacture and commissioning

We present details of the technical design, manufacture and testing of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase xenon detector which measures both the scintillation light and the ionisation charge generated in the liquid by interacting particles and radiation. The instrument design is driven by both the physics requirements and by the technology requirements surrounding the use of liquid xenon. These include considerations of key performance parameters, such as the efficiency of scintillation light collection, restrictions placed on the use of materials to control the inherent radioactivity levels, attainment of high vacuum levels and chemical contamination control. The successful solution has involved a number of novel design and manufacturing features which will be of specific use to future generations of direct dark matter search experiments as they struggle with similar and progressively more demanding requirements.

Nuclear recoil scintillation and ionisation yields in liquid xenon from ZEPLIN-III data

Scintillation and ionisation yields for nuclear recoils in liquid xenon above 10 keVnr (nuclear recoil energy) are deduced from data acquired using broadband Am–Be neutron sources. The nuclear recoil data from several exposures to two sources were compared to detailed simulations. Energy-dependent scintillation and ionisation yields giving acceptable fits to the data were derived. Efficiency and resolution effects are treated using a light collection Monte Carlo, measured photomultiplier response profiles and hardware trigger studies. A gradual fall in scintillation yield below ∼40 keVnr is found, together with a rising ionisation yield; both are in agreement with the latest independent measurements. The analysis method is applied to the most recent ZEPLIN-III data, acquired with a significantly upgraded detector and a precision-calibrated Am–Be source, as well as to the earlier data from the first run in 2008. A new method for deriving the recoil scintillation yield, which includes sub-threshold S1 events, is also presented which confirms the main analysis.

Prospects for observation of neutrino-nuclear neutral current coherent scattering with two-phase Xenon emission detector

We propose to detect and to study neutrino neutral current coherent scattering off atomic nuclei with a two-phase emission detector using liquid xenon as a working medium. Expected signals and backgrounds are calculated for two possible experimental sites: the Kalinin Nuclear Power Plant in the Russian Federation and the Spallation Neutron Source at the Oak Ridge National Laboratory in the U.S.A. Both sites have advantages as well as limitations. The experiment looks feasible at either location.

Limits on inelastic dark matter from ZEPLIN-III

Abstract We present limits on the WIMP–nucleon cross section for inelastic dark matter from a reanalysis of the 2008 run of ZEPLIN-III. Cuts, notably on scintillation pulse shape and scintillation-to-ionisation ratio, give a net exposure of 63 kg day in the range 20– 80 keV nuclear recoil energy, in which 6 events are observed. Upper limits on signal rate are derived from the maximum empty patch in the data. Under standard halo assumptions a small region of parameter space consistent, at 99% CL, with causing the 1.17 ton yr DAMA modulation signal is allowed at 90% CL: it is in the mass range 45– 60 GeV c − 2 with a minimum CL of 87%, again derived from the maximum patch. This is the tightest constraint yet presented using xenon, a target nucleus whose similarity to iodine mitigiates systematic error from the assumed halo.

The ZEPLIN-III dark matter detector: Performance study using an end-to-end simulation tool

We present results from a GEANT4-based Monte Carlo tool for end-to-end simulations of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase detector which measures both the scintillation light and the ionisation charge generated in liquid xenon by interacting particles and radiation. The software models the instrument response to radioactive backgrounds and calibration sources, including the generation, ray-tracing and detection of the primary and secondary scintillations in liquid and gaseous xenon, and subsequent processing by data acquisition electronics. A flexible user interface allows easy modification of detector parameters at run time. Realistic datasets can be produced to help with data analysis, an example of which is the position reconstruction algorithm developed from simulated data. We present a range of simulation results confirming the original design sensitivity of a few times 10−8 pb to the WIMP-nucleon cross-section.

Experimental methods for particle dark matter detection (review)

This review presents the results of earlier studies and new designs at different stages of development, which are aimed at searching for particle Dark Matter.

Condensed krypton scintillators

Abstract Liquid and solid krypton have been studied as scintillators. Attenuation length for Kr scintillation light was determined to be about 1m, after the Kr was purified by a hot Ca-getter and by a Ti discharge purifier in liquid phase. Two detectors (75 and 51 in size, respectively) were tested as stop-counters in a time-of-flight experiment using 1–2 GeV/ c particle beams at the ITEP accelerator. A FWHM time resolution of ~2 ns was achieved. Using Si-PhD to detect the scintillation light in liquid and solid Kr. resolution of 16% (FWHM) for α( 239 Pu)-scintillation was achieved in both liquid and solid Kr.

Radioactivity backgrounds in ZEPLIN-III

We examine electron and nuclear recoil backgrounds from radioactivity in the ZEPLIN-III dark matter experiment at Boulby. The rate of low-energy electron recoils in the liquid xenon WIMP target is 0.75±0.05 events/kg/day/keV, which represents a 20-fold improvement over the rate observed during the first science run. Energy and spatial distributions agree with those predicted by component-level Monte Carlo simulations propagating the effects of the radiological contamination measured for materials employed in the experiment. Neutron elastic scattering is predicted to yield 3.05±0.5 nuclear recoils with energy 5–50 keV per year, which translates to an expectation of 0.4 events in a 1-year dataset in anti-coincidence with the veto detector for realistic signal acceptance. Less obvious background sources are discussed, especially in the context of future experiments. These include contamination of scintillation pulses with Cherenkov light

Performance of the veto detector incorporated into the ZEPLIN-III experiment

The ZEPLIN-III experiment is operating in its second phase at the Boulby Underground Laboratory in search of dark matter WIMPs. The major upgrades to the instrument over its first science run include lower background photomultiplier tubes and installation of a plastic scintillator veto system. Performance results from the veto detector using calibration and science data in its first six months of operation in coincidence with ZEPLIN-III are presented. With fully automated operation and calibration, the veto system has maintained high stability and achieves near unity live time relative to ZEPLIN-III. Calibrations with a neutron source demonstrate a rejection of 60% of neutron-induced nuclear recoils in ZEPLIN-III that might otherwise be misidentified as WIMPs. This tagging efficiency reduces the expected untagged nuclear recoil background from neutrons during science data taking to a very low rate of ≃0.2 events per year in the WIMP acceptance region. Additionally, the veto detector provides rejection of 28% of γ-ray induced background events, allowing the sampling of the dominant source of background in ZEPLIN-III – multiple scatter γ-rays with rare topologies. Since WIMPs will not be tagged by the veto detector, and tags due to γ-rays and neutrons are separable, this population of multiple scatter events may be characterised without biasing the analysis of candidate WIMP signals in the data.