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 3.9E-8 pb
for a 52 GeV WIMP. A WIMP-neutron result is also shown, which bottoms out
at 8.0E-3 pb for a
50 GeV WIMP.
SPIN-INDEPENDENT WIMP-NUCLEON CROSS-SECTION
(data for ZEPLIN-III
combined result here; Akimov et al. (2012), Phys. Lett. B 709: 14;
arXiv:1110.4769)
SPIN-DEPENDENT
WIMP-NEUTRON CROSS-SECTION
(data
for ZEPLIN-III combined result here;
Akimov et al. (2012), Phys. Lett. B 709: 14;
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 27th February 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 8.1E-8 pb
for a 60 GeV WIMP mass. This surpasses our sensitivity prediction of
around 1E-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 1.9E-2 pb at 55 GeV WIMP mass. Recent
calculations of the nuclear spin structure based on the Bonn CD nucleon-nucleon
potential were used for the odd-neutron isotopes Xe-129 and Xe-131. 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, 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.
