The ZEPLIN dark matter search: two phase xenon as a WIMP target

Abstract

The existence of a significant non-baryonic component to the universe is widely accepted, with worldwide efforts underway trying to detect this so-called dark matter. The ZEPLIN detectors utilise liquid xenon as a target medium in the search of the expected rare interactions of Weakly Interacting Massive Particles, or WIMPs, with ordinary baryonic matter. The neutralino, arising in supersymmetric extensions to the standard model of particle physics, provides a particularly well-motivated candidate. The ZEPLIN-II and ZEPLIN-III experiments, operate in two-phase mode (liquid/gas), measuring both the scintillation and ionisation signatures produced during an interaction. These instruments form the basis of this thesis. The ZEPLIN-II experiment was operated underground at the Boulby Underground Laboratory, culminating in a WIMP search run lasting 57 days. Some key operational aspects are discussed, and a full description of the data analysis is given, which yielded a competitive upper limit on the spin-independent WIMP-nucleon scattering crosssection with a minimum of 6.6×10−7 pb for a 65 GeV/c2 WIMP with 90% confidence. Subsequently, a smaller collaboration proceeded with the commissioning and operation of ZEPLIN-III at Boulby. The detector was operated stably for 12 months, culminating in the first science run, which excluded a cross-section above 7.7 × 10−8 pb for a 55 GeV/c2 WIMP. This placed ZEPLIN-III as one of the world’s leading WIMP search experiments. Along with the WIMP search results, the data collected from these instruments have been exploited to extract information about the underlying xenon physics processes, which will play an important role in design of future systems. This includes the first quantitative measurements of single electron emission in a two-phase noble gas detector, studies of the field dependence of their response and of the anti-correlation between the scintillation and ionisation channels.