The NAIAD experiment for WIMP searches at Boulby mine and recent results
Introduction
The UK Dark Matter Collaboration (UKDMC) has been operating NaI(Tl) detectors at the Boulby Mine underground site for several years [1]. Limits on the flux of weakly interacting massive particles (WIMPs), that may constitute up to 90% of the mass of the Galaxy, have been published using the data from the first encapsulated detector [2], [3]. Pulse shape analysis (PSA) has been applied to the data to distinguish between slow scintillations arising from background electron recoils and fast scintillations due to nuclear recoils, which are expected from WIMP–nucleus interactions [4]. Improved limits were then obtained by the DAMA experiment, which also used NaI(Tl) scintillation detectors with PSA and had larger statistics [5].
Since then, the DAMA group moved to a simple annual modulation analysis of the background rate in its crystals without using PSA. The group claims that it observes an annual modulation consistent with the expected signal from WIMP–nucleus interactions with a specific set of WIMP parameters [6].
Although several existing experiments have a potential to probe the whole region of WIMP parameters allowed by the DAMA signal (see, for example, Refs. [7], [8], [9], [10] and Ref. [11] for recent review), they use other techniques and other target materials. This leaves room for speculation about possible uncertainties in the comparison of results. These uncertainties are related to systematic effects and nuclear physics calculations. Running an experiment, NAIAD, with the same target (NaI) and detection technique but different analysis would help in the understanding of possible systematic effects. Such an experiment will also be complementary to more sensitive detectors in studying regions of WIMP parameter space favoured by the DAMA positive signal.
The advance of the UKDMC NaI(Tl) experiment has been blocked for a few years by the discovery of a fast anomalous component in the data from several encapsulated crystals [1]. These events were faster than typical electron recoil pulses and faster even than nuclear recoil pulses [1], [12]. Similar events have also been seen by the Saclay group [13]. The most plausible explanation of these events at the moment is implanted surface contamination of the crystal by an alpha-emitting isotope from radon decay [14], [15].
NAIAD was constructed with unencapsulated crystals to allow better control of the crystal surface, to reduce the background due to such events and to improve the light collection [16]. Such an experiment could probe the region of WIMP parameters allowed by the DAMA experiment using a similar detection technique and target, and reach the region favoured by some SUSY models [16]. NaI has an advantage of having two targets with high and low masses, thus reducing uncertainties related to nuclear physics calculations. The detectors are sensitive to both spin-independent and spin-dependent interactions. The experiment is complementary to other dark matter experiments at Boulby, such as ZEPLIN [10] (xenon gas and double-phase xenon detectors) and DRIFT [17] (time projection chamber with directional sensitivity). The array of NaI(Tl) detectors can also be used as a diagnostic array to study background and systematic effects for other experiments at Boulby. (Note that the new data from this array are indeed free from an anomalous fast component [15].)
In this paper we present the status of the NAIAD array (NaI Advanced Detector) at Boulby. Section 2 describes the experimental set-up. The analysis procedure, various calibrations and their results are presented in Section 3. WIMP limits from data collected up to the end of 2001 are shown in Section 4. Our conclusions are given in Section 5.
Section snippets
NAIAD experiment
The NAIAD array is operational in the underground laboratory at Boulby mine (North Yorkshire, UK) at a vertical depth of about 1100 m. In its final stage the NAIAD array will consist of eight NaI(Tl) crystals from different manufacturers (Bicron, Hilger, VIMS). At present (May 2002) six detectors are running with a total mass of 46 kg. Two detectors contain encapsulated crystals, while four other crystals are unencapsulated. To avoid their degradation by humidity in the atmosphere, the
Analysis procedure and calibrations
Final analysis has been performed on the sum of the pulses from the two PMTs attached to each crystal. The parameters of the pulses from each PMT have been used to apply so-called asymmetry cuts (described below) to remove those events with obvious asymmetry between the pulses from each PMT. Our standard procedure of data analysis involves fitting of a single exponential to each integrated pulse in order to obtain the index of the exponent, τ, the amplitude of the pulse, A, and the start time, t
Results and discussion
The UK Dark Matter Collaboration has been operating the detectors of the NAIAD array since the winter of 2000. The first crystal (DM74) was immersed in pure mineral oil and ran for more than six months until degradation of the crystal surface resulted in a significant decrease of the light yield. The second crystal (DM72) was installed in summer of 2000 in a sealed copper box and has been running since then with a loss of light yield of no more than 10%. The data collected with these two
Conclusions
The status of the NAIAD experiment for WIMP dark matter search at Boulby mine has been presented. The detector consists of an array of NaI(Tl) crystals with high light yield. Currently six crystals are collecting data. PSA has been used to discriminate between nuclear recoils, possibly caused by WIMP interactions, and electron recoils due to gamma-ray background. We have presented upper limits on the WIMP–nucleon spin-independent and WIMP–proton spin-dependent cross-sections based on the data
Acknowledgements
The Collaboration wishes to thank PPARC for financial support. We are grateful to the staff of Cleveland Potash Ltd. for assistance. We also thank the referee for very useful comments.
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Now at the University of Pennsylvania, Philadelphia, PA 19104, USA.