1. ZEPLIN I: First limits on nuclear recoil events Vitaly A. Kudryavtsev Department of Physics and Astronomy University of Sheffield, UK For the UK Dark Matter Collaboration: University of Edinburgh Imperial College London CCLRC Rutherford Appleton Laboratory University of Sheffield IDM2004, 6-10 September, 2004

2. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Outline Boulby Underground Laboratory and dark matter searches Detection principle in xenon ZEPLIN I detector and DAQ Calibrations, cuts and cut efficiencies Data and analysis Results and limits Summary and conclusions

3. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh The Boulby Underground Laboratory Boulby Mine - operated by Cleveland Potash Ltd. Vertical depth - 1070 m Rock around the lab - halite Effective column density at vertical (flat surface) - 2805±45 m w. e. Muon flux (measurements) - (4.09±0.15)  10 -8 cm -2 s -1 (Robinson et al. NIMA 511 (2003) 347. Radioactivity (measurements) - about 70 ppb U, 125 ppb Th and 1130 ppm of K (Smith et al., in preparation). Neutron flux (simulations) - about 2  10 -6 cm -2 s -1 above 100 keV and 10 -6 cm -2 s -1 above 1 MeV (without an effect of back-scattering from the walls) (Carson et al. Astrop. Phys., in press) - more at the parallel session on backgrounds - Thursday afternoon. More on the Boulby Undergound Laboratory on Friday (A. Murphy)

4. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Dark matter searches at Boulby ZEPLIN - ZonEd Proportional scintillation in LIquid Noble gases (or Zoned Electro-luminescence and Primary Light In Noble Gases) - liquid (ZEPLIN I) and two-phase (ZEPLIN II/III/MAX) Xe detectors (see also talk by H. Wang on ZEPLIN II and E. Daw on large-scale xenon programme) DRIFT - Directional Recoil Identification From Tracks - low pressure gas Time Projection Chamber (see talk on DRIFT by N. Spooner) NAIAD - NaI Advanced Detector - array of NaI(Tl) crystals - almost completed; resources moved to other experiments (see poster)

5. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Detection principle Excitation –production and decay of excited Xe 2 * states, which decay through singlet (3 ns) and triplet (27 ns) modes, emitting 175 nm photons. –dE/dx of the recoil determines the proportion of energy channelled into these two decay modes: the ratio of singlet to triplet decays is a few times higher for nuclear recoils than for electron recoils. Ionisation –ionised state Xe 2 +, which can recombine with the liberated electron giving Xe 2 * state decaying as described above. –dE/dx of the incident particle determines the recombination time. For nuclear recoils the recombination occurs within picoseconds of the interaction, for electron recoils the recombination time is of the order of 40 ns. Nuclear recoil pulses are faster than electron recoil ones. The recombination can be suppressed by an electric field and the ionisation yield can be directly measured (ZEPLIN II/III - see H. Wang talk on ZEPLIN II, Wednesday) Davies et al. Phys. Lett. B 320 (1994) 395

6. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Detection principle and ZEPLIN I Recombination is allowed to occur (no electric field) - ZEPLIN I. –Only scintillation light, which includes components from excitation and recombination. –Discrimination between nuclear and electron recoil - pulse shape analysis, based on the difference in the time constant. Active veto - 0.93 tonne of liquid scintillator Vacuum and target vessels (copper - low radioactivity) PMTs

7. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Detector

8. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Detector and DAQ Total xenon mass - 5 kg fiducial mass - 3.2 kg Mean event rate: 2 Hz Trigger: 3-fold coincidences at 1 p.e. 2 keV software threshold Light yield: 1.5-2.5 p.e./keV Active veto - 30 cm thick - to reject gamma background from PMTs (mainly), also passive shielding against rock neutrons Lead castle - 25 cm thick - against rock gammas (and neutrons) Statistics: 293 kg  days (3 runs) Pb shielding Top of ZEPLIN I veto Boulby stub 2 laboratory xenon purification

9. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh DAQ pulses Pulses from 3 PMTs + sum pulse + veto PMT pulses integrated using integrating buffer Digitised using dual range 8bit digitisers: –LeCroy DSO (first runs) –Compact PCI Acqiris (later on) Fitted to exponential to guide eye; several pulse shape estimates stored: time constant, mean time, median time etc. Mean time of the pulse is used for final pulse shape analysis Time / ns PMT 1 PMT 3 PMT 2 SUM

10. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Energy calibration 122 keV peak 30 keV X-ray 92 keV recoil e- Linear response ( 57 Co calibration is effective point source) Energy resolution 57 Co Calibration 136 keV peak (10%)

11. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Energy calibration and light collection Geant4 simulation of 57 Co calibration with/without energy resolution Calibration with various sources. Variation in yield is due to a non-uniform response. Light collection matrix, which describes the efficiency of light collection (20-25% at 662keV)

12. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Fiducial volume cut Single PMT event Noise trigger Bulk events Turret events Fiducial volume cut Projection of normalised amplitudes from PMTs onto a plane Double PMT event Simulations of light collection using Guideit allows calculation of light collection matrix and volume cut efficiencies

13. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Cuts and efficiencies Trigger efficiency: simulations cross-checked with data from a special run with a gamma source (flat Compton spectrum is expected if the trigger efficiency is 100%). Volume cut efficiency: simulations (confirmed by the calibration measurements with various sources). Noise cut: reject fast noise events 2.5 in any PMT. Efficiency: simulations cross- checked with calibration data. Amplitude, V Number of events

14. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh ZEPLIN I: surface neutron calibrations ZEPLIN I mean time (  ) distributions are fitted to  (gamma) density function in 1/  Neutron source calibration down to 3 keV Ambient neutron calibration (at the surface) at 3-10 keV Small statistics - we use conservatively the ratio of mean time constants R  =0.5 (an upper limit to the ratio). 3-7 keV: neutrons (ambient) + gammas R  =  n /   =0.42±0.07 3-8 keV: neutrons (source) + gammas R  =  n /   =0.44±0.09 20-30 keV: neutrons (source) + gammas R  =  n /   =0.64±0.04

15. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Gamma calibration and data Poisson F-C Underground gamma calibration with gamma source (also vetoed events) Underground data No 2nd component was seen in gamma calibration runs or data runs - no nuclear recoil observed. Analysis: 1 - is signal observed (slopes - next slide)? 2 - limits: deviation from gamma function at left hand side of the data distribution - Poisson analysis

16. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Data analysis Zero signal from slope of left branch

17. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Energy spectra and limits (preliminary) on nuclear recoils S1: raw spectrum S2: cut spectrum F1: reconstructed spectrum without trigger efficiency L1: nuclear recoil limit L2: nuclear recoil limit with efficiency correction June 2002 data run

18. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Calculation of limits Limits on nuclear recoils - Poisson analysis (deviation from gamma function) on the left hand side of the time constant distribution of data. Correction for a non-observed part of the distribution (hidden under the distribution of gamma events) - assuming ratio of 0.5 (from neutron and gamma calibrations - conservative approach). Correction for trigger, volume cut and noise cut efficiencies. Energy resolution and light collection matrix. Quenching factor = 0.22 (from D. Akimov et al., Phys. Lett. B 524 (2002) 245-251). Standard halo model: spherical, isothermal, Maxwellian velocity distribution with v 0 =220 km/s, density 0.3 GeV/cm 3.

19. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh ZEPLIN I limits (preliminary) Spin-independent interactions

20. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh ZEPLIN I limits (preliminary) Spin-dependent interactions Spin factors and form factors from Ressell and Dean, Phys. Rev. C 56 (1997) 535, Bonne-A potential, higgsino type WIMPs (important for form-factors only) - method from Tovey et al. Phys. Lett. B, 488 (2000) 17.

21. 7 September, 2004Vitaly Kudryavtsev, Sheffield, UKIDM2004, Edinburgh Summary and Conclusions 293 kg  days of data have been collected with the ZEPLIN I liquid xenon detector. Pulse shape analysis has been used to discriminate between nuclear (potential WIMP signal) and electron (gamma background) recoils. Neutron and gamma calibrations have been performed. Extensive studies of the detector, its energy resolution, light collection, cut efficiencies have been done. As a result, one of the world best limits (still preliminary) has been obtained with ZEPLIN I. The UK Dark Matter Collaboration together with its oversea partners (US, Europe, Russia) is moving to two-phase xenon detectors ZEPLIN II/III/… and detectors with directional sensitivity DRIFT.