ZEPLIN-III SECOND SCIENCE RUN RESULTS

The second science run of ZEPLIN-III at Boulby produced 1,344 kg·days of fiducial exposure collected from 319 days of continuous operation between June 2010 and May 2011. The final result indicates a most likely WIMP signal of zero, with a 90% confidence limit of 5.1 events over 251 kg·days of effective fiducial exposure (50 GeV WIMP average). First run data were re-analysed with the updated software, producing an upper limit of 4.2 signal events in 107 kg·days effective exposure. Combined results for the two runs of the experiment are shown below (6.0 events in the aggregate exposure at 90% CL). The spin-independent interaction is excluded with 90% confidence as show in the figure below, which has a minimum of 3.9x10^-8 pb for a 52 GeV/c² WIMP. A WIMP-neutron result is also shown, which bottoms out at 8.0x10^-3 pb for a 50 GeV WIMP

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SPIN-INDEPENDENT WIMP-NUCLEON CROSS-SECTION

(data for ZEPLIN-III combined result here; Akimov et al. 2011, arXiv:1110.4769)

SPIN-DEPENDENT WIMP-NEUTRON CROSS-SECTION

(data for ZEPLIN-III combined result here; Akimov et al. 2011, arXiv:1110.4769)

ZEPLIN-III FIRST SCIENCE RUN RESULTS

SPIN-INDEPENDENT WIMP-NUCLEON CROSS-SECTION

(Lebedenko et al. 2009, Phys. Rev. D 80: 052010)

In 2009 the ZEPLIN-III Collaboration announced the result of the first science run of ZEPLIN-III at the Boulby Underground Laboratory. 850 kg*days of raw data were collected over an 83-day period beginning on 27^th Feb 2008, with extensive calibrations undertaken before and after the ‘dark’ run. The final result indicates a most likely WIMP signal of zero, with a 90% confidence limit of 3.05 events over 128 kg*days of effective fiducial exposure. The WIMP-nucleon spin-independent interaction is excluded at 90% level as show in the figure below, which has a minimum of 8.1x10^-8 pb for a 60 GeV/c² WIMP mass. This surpasses our sensitivity prediction of around 1x10^-7 pb for phase I of the experiment.

SPIN-DEPENDENT WIMP-NUCLEON CROSS-SECTION

(Lebedenko et al. 2009, Phys. Rev. Lett. 103: 151302)

Based on the analysis leading to the spin-independent result, ZEPLIN-III can exclude, with 90% confidence, a WIMP-neutron cross-section greater than 1.9x10^-2 pb at 55 GeV/c² WIMP mass. Recent calculations of the nuclear spin structure based on the Bonn CD nucleon-nucleon potential were used for the odd-neutron isotopes ¹²⁹Xe and ¹³¹Xe. These indicate that the sensitivity of xenon targets to the spin-dependent WIMP-proton interaction is much lower than implied by previous calculations, whereas the WIMP-neutron sensitivity is impaired only by a factor of ~2.

INELASTIC DARK MATTER RESULTS

(Akimov et al., 2010, Phys. Lett. B 692: 180)

WIMP dark matter scattering predominantly into a higher-mass state has been proposed as an explanation of the annually-modulated event rate in DAMA which is also consistent with the upper limits on WIMP-nucleon elastic scattering rates from other experiments. A search of 63 kg·days net exposure yielded 6 candidate events in the range 20–80 keV nuclear recoil energy. They were consistent, both in number and scintillation-to-ionisation ratio, with belonging to the tail of the electron recoil background population. Single-sided upper limits were set on the WIMP-nucleon cross section, constraining the DAMA-explaining region of IDM parameter space: for a standard halo model there remains a 90%-CL allowed region for WIMP masses in the range 45–60 GeV/c², with minimum CL 88%. In particular, a target element of similar mass to DAMA’s iodine reduces systematic uncertainty due to the WIMP velocity distribution.