Dark Matter Research


The ZEPLIN I detector was a single phase liquid xenon detector of 3.1kg fiducial volume, viewed by three photomultipliers through silica windows and optically isolated, self shielding, liquid xenon turrets. Event classification was provided through pulse shape discrimination. The target was enclosed by a multi-purpose, 1 tonne, PXE-based liquid scintillator shield and an outer passive lead shield. The liquid scintillator shield acted as a veto for PMT events and also provided a Compton gamma calibration contemporaneous with the data collected, an active shield for external gammas and a high purity inner shield. The combined data set from ZEPLIN I comprised 293 kg.days of dark matter data, with contemporaneous gamma calibrations and an extended gamma calibration and an efficiency test run. In later stages there were problems refilling the target. An underground neutron calibration run was planned, but stalled, as the technical difficulty with the target remained unresolved. Only a small amount of xenon could be condensed within the system (about 1kg), even though 'fixed bulb' tests indicated that pressure and temperature readings are accurate and the gas accurately tracked the SVP curves. The oxysorb had been replaced within the purification system, both chamber and purifier cleaned and pumped on for extended periods, the cryogenic systems cleaned and recharged, vacuum chamber opened and PMTs removed to ensure the internal chamber was in good condition. Given the programme priorities and requirements to progress the two phase xenon detectors, effort was reluctantly re-allocated to the ZEPLIN II/III programme and plans for relocating the required ZEPLIN I infrastructure for ZEPLIN II put in place.

Combined Data Sets

The combined data set of dark matter runs for ZEPLIN I comprised 293 kg.days of data; 25 days from November 2001, 50 days from June 2002 and 16 days from October 2002. The light yield for the final run in October 2002 reached 2.5 p.e./keV, which gave a hardware threshold below 2 keV (3 p.e.) The October 2002 run was also accompanied by an extended Compton calibration using an external gamma source, which augments the lower statistic internal calibrations using the Compton veto triggered events. Additionally, the October 2002 data set was collected using xenon purchased with tighter specifications on the krypton impurity. The rate at 100keV showed a three fold reduction, supporting the hypothesis that some component of the background in the earlier runs is due to the 85Kr beta emission.

Analysis Technique

Zeplin I Analysis
ZEPLIN I Upper Limit

The analysis techniques applied to the data were rechecked, with several internal analyses being undertaken to check time constant ratios, efficiencies and pulse shape distributions. Efficiencies of PMT noise cuts, triggering efficiencies and light collection variations were fully calculated and, where possible, cross checked against data. A dedicated trigger efficiency run had been performed in December 2004 with a partly filled chamber to provide checks on electronic efficiencies. Re-analysis of the surface neutron calibration data was performed by several independent routes and consistent results attained. A Feldman-Cousins based analysis was introduced to calculate the 90% upper limit to the nuclear recoil signal that could be present within the data set. Figure 2 1 shows the limit obtained compared to the DAMA annual modulation signal for a common halo model and full spin independent interaction. The robustness of the ZEPLIN I result was assessed by a detailed study of the systematics within the data sets and of the assumptions made during the analysis. Key assumptions identified were the time constant ratio between neutron and gamma events, the quenching factor and the effect of the non uniform light collection within the target. The latter was important as the light yield from the target improved during the lifetime of the experiment, believed to be due to self cleansing of the target by the xenon. The quenching factor was held constant in the ZEPLIN I analysis at 0.22, the UKDM measured value (other measurements in the literature are 0.2±0.05 (UCLA), 0.3±0.1 (XENON) and 0.5±0.05 (DAMA)). Measurements of the time constant ratio by exposing the ZEPLIN I detector to ambient and laboratory neutron sources showed a decrease from the value 0.64±0.04 at 20 keV to the value 0.43±0.06 at 3-7 keV electron-equivalent energy. The analysis used a conservative assumption that the ratio remained constant at 0.50, given the statistical error in the neutron calibration runs. If the central values were used, suggesting a possible decrease of the time constant ratio, a repeat of the complete analysis gives a limit lower by a factor 1.4.

The paper submitted to Astroparticle Physics was accepted for publication and finally appeared in June 2005. The upper limit in that paper was essentially unchanged from our first conference announcement in late 2002 and the subsequent preprint released in October 2004. A critique of the ZEPLIN I result was published by a rival European cryogenic group in June 2006. In response to this we documented in more detail some of the systematic study work that had been done specifically for this purpose and this was published in the same refereed literature as the critique in November 2006. The community at large is routinely referencing the ZEPLIN I result and not the critique and the ZEPLIN I result is vindicated.

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