Murphy, Alex

Ghag, Chamkaur

Reichhart, Lea

2013-10-17T14:39:00Z

2013-10-17T14:39:00Z

2013-11-28

Astrophysical observations give convincing evidence for a vast non-baryonic component, the so-called dark matter, accounting for over 20% of the overall content of our Universe. Direct dark matter search experiments explore the possibility of interactions of these dark matter particles with ordinary baryonic matter via elastic scattering resulting in single nuclear recoils. The ZEPLIN-III detector operated on the basis of a dualphase (liquid/gas) xenon target, recording events in two separate response channels { scintillation and ionisation. These allow discrimination between electron recoils (from background radiation) and the signal expected from Weakly Interacting Massive Particle (WIMP) elastic scatters. Following a productive first exposure, the detector was upgraded with a new array of ultra-low background photomultiplier tubes, reducing the electron recoil background by over an order of magnitude. A second major upgrade to the detector was the incorporation of a tonne-scale active veto detector system, surrounding the WIMP target. Calibration and science data taken in coincidence with ZEPLIN-III showed rejection of up to 30% of the dominant electron recoil background and over 60% of neutron induced nuclear recoils. Data taking for the second science run finished in May 2011 with a total accrued raw fiducial exposure of 1,344 kg days. With this extensive data set, from over 300 days of run time, a limit on the spin-independent WIMP-nucleon cross-section of 4.8 10-8 pb near 50 GeV/c2 WIMP mass with 90% confidence was set. This result combined with the first science run of ZEPLIN-III excludes the scalar cross-section above 3.9 10-8 pb. Studying the background data taken by the veto detector allowed a calculation of the neutron yield induced by high energy cosmic-ray muons in lead of (5.8 0.2) 10-3 neutrons/muon/(g/cm2) for a mean muon energy of 260 GeV. Measurements of this kind are of great importance for large scale direct dark matter search experiments and future rare event searches in general. Finally, this work includes a comprehensive measurement of the energy dependent quenching factor for low energy nuclear recoils in a plastic scintillator, such as from the ZEPLIN-III veto detector, increasing accuracy for future simulation packages featuring large scale plastic scintillator detector systems.

http://hdl.handle.net/1842/7914

en

The University of Edinburgh

L. Reichhart et al. Measurement and simulation of the muon-induced neutron yield in lead. arXiv:1302.4275v1 [physics.ins-det], submitted to Astroparticle Physics, 2013.

L. Reichhart et al., Quenching Factor for Low Energy Nuclear Recoils in a Plastic Scintillator. In Physical Review C 85:065801, 2012.

dark matter

ZEPLIN-III

WIMPs

liquid xenon detectors

veto detector

plastic scintillator

quenching

cosmic ray muons

neutrons

Monte Carlo simulations

ZEPLIN-III direct dark matter search : final results and measurements in support of next generation instruments

Thesis or Dissertation

Doctoral

PhD Doctor of Philosophy