Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment

TL;DR

This paper reports the results from the first science run of the ZEPLIN-III dark matter search, using a 12 kg two-phase xenon TPC to search for WIMPs via nuclear recoils. It details a thorough calibration program (57Co, AmBe, and Cs-137) to characterize light yield, energy response, and detector stability, enabling robust S1/S2 discrimination and energy reconstruction. An 83-day science run yielded 847 kg·days of data with 7 events in the predefined WIMP-search region, leading to a conservative 90% C.L. upper limit of $8.1\times 10^{-8}$ pb on the WIMP-nucleon cross-section at $m_\chi = 60~\mathrm{GeV}/c^2$, and demonstrating superior low-energy discrimination relative to prior xenon experiments. A notable finding is a mismatch between the AmBe-calibrated nuclear recoil spectrum and Monte Carlo simulations, which is addressed by invoking a non-linearity in the low-energy recoil energy scale, a consideration that informs the interpretation of the limit and underscores the need for deeper understanding of xenon detector response at low energies.

Abstract

The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12kg two-phase xenon time projection chamber to search for the weakly interacting massive particles (WIMPs) that may account for the dark matter of our Galaxy. The detector measures both scintillation and ionisation produced by radiation interacting in the liquid to differentiate between the nuclear recoils expected from WIMPs and the electron recoil background signals down to ~10keV nuclear recoil energy. An analysis of 847kg.days of data acquired between February 27th 2008 and May 20th 2008 has excluded a WIMP-nucleon elastic scattering spin-independent cross-section above 8.1x10(-8)pb at 55GeV/c2 with a 90% confidence limit. It has also demonstrated that the two-phase xenon technique is capable of better discrimination between electron and nuclear recoils at low-energy than previously achieved by other xenon-based experiments.

Figures (17)

• Figure 1: Segment of the high-sensitivity summed waveform for a neutron elastic scattering event with S1$= 5~$keVee, showing a small primary pulse (S1) preceding a large secondary pulse (S2). Some PMT after-pulsing and, possibly, single electron emission can be seen following S2. Note that only excursions $>$3 rms on individual channels are added into the summed waveform. See later text for more detailed discussion of some of these points.
• Figure 2: Response to an external $^{57}$Co $\gamma$-ray source in the combined energy channel, exploiting S1 and S2 anti-correlation. One day's experimental data are shown in blue with statistical error bars. The simulation result is indicated in red: the solid histogram shows the bare energy deposits and the shaded one shows the result of Gaussian-smearing with the energy resolution indicated in the figure.
• Figure 3: Distribution in the horizontal plane of events from the $^{57}$Co source on the left and from the AmBe source on the right. The source positions are different for each image and neither is centred. In both cases the volume distribution is as expected from Monte Carlo simulations, given the location of each source. Interaction vertices can be seen out to the edge of the fiducial volume at a radius of 150 mm (red circle). The outer circle shows the edge of the liquid xenon target. Each PMT is marked by two smaller circles (PMT centres and envelopes).
• Figure 4: Expected mean number of S1 photoelectrons as a function of the mean pulse area observed in the central channel in the array. The expected signal is the mean of the Poisson distribution obtained by counting the frequency of 'zeros', i.e. the absence of any response.
• Figure 5: Calibration of the nuclear recoil response with an AmBe neutron source, plotted as the discrimination parameter ($\log_{10}\left(S2/S1\right)$) as a function of 'electron-equivalent energy' (i.e. using the S1 channel calibrated by $^{57}$Co). The lines show the trends of the mean and standard deviation of energy-binned log-normal fits to the recoil population. The distinct population above $\sim$40 keVee is due to inelastic neutron scattering off $^{129}$Xe nuclei.