ZEPLIN-III Second Science Run &Beyond& Beyond Henrique Araújo Imperial College London On behalf of the ZEPLIN-III Collaboration: Edinburgh University (UK), Imperial College London (UK), ITEP-Moscow (R(Ri)ussia), LIP-CiCoim bra (Pt(Portuga l) STFC Rutherford Appleton Laboratory (UK) Darkness Visible, 2-6 August 2010, IOA Cambridge Direct WIMP detection: how hard can it be? { You are looking at a snapshot of n-body simulations of the Milky Way’s DM halo (Via Lactea I) 2 The exppgerimental challenge { Low-energy particle detection is easy ;) E.g. Microcalorimetry with superconducting TES Detection of keV particles/photons with eV FWHM! { Rare event searches are also easy ;) E.g. Super-Kamiokande contains 50 kT water Cut to ~20 kT fiducial mass (self-shielding) { But doing both is hard! Small is better for signal collection Large is better for shielding background { Doh: andhd there is no trigger, and no control of luminosity… Low energy nuclear recoils { Elastic scatter off nucleus: F N o F N z Decreasing, featureless spectrum of low-energy recoils (<~100 keV) z Rate depends on target mass & spin, WIMP mass & spin, DM halo, … z Neutrons are irreducible background * { Inelastic scatter off nucleus: F N o F N o F N J z Short-lived, low-lying excited states (easier signature?) z 129Xe(3/2+ĺ1/2+) + J (40 keV), 73Ge(5/2+ĺ9/2+) + J (13 keV) z Neutrons are irreducible background * { Inelastic dark matter (iDM): F N o F N z If there is no elastic channel, then kinematic effects may ensure that “particles will scatter at DAMA but not at CDMS” (Smith & Weiner 2001) z Recoil spectrum with threshold (mass splitting, G) z Neutrons are irreducible background The WIMP-search technology zoo Ionisation Detectors Targets: Ge, Si, CS 2, CdTe CoGeNT, DRIFT, GENIUS, Light & Ionisation HDMS, IGEX, NEWAGE Heat & Ionisation Detectors BlBolometers Targets: Xe, Ar Targets: Ge,Si ionisation ArDM, LUX, WARP, CDMS, EDELWEISS XENON, ZEPLIN Q cryogeni(50ic (<50 mK) cold (LN2) SitilltScintillators BlBolome ters Targets: NaI, Xe, Ar Targets: Ge, Si, Al 2O3, TeO2 ANAIS, CLEAN, DAMA, CRESST-I, CUORE, CUORICINO DEAP, KIMS, LIBRA, NAIAD, XMASS, ZEPLIN-I Bubbles & Droplets Light & Heat Bolometers CF3Br, CF3I, C3F8, C4F10 Targets: CaWO 4, BGO, Al2O3 COUPP, PICASSO , SIMPLE CRESST, ROSEBUD cryogenic (<50 mK) ZEPLIN-III: 2 -phase xenon TPC Readout of scintillation light and iitiionisation charge with array of 31 photomultipliers Gaseous xenon Liquid xenon photomultipliers Sensitivity to single ionisation electrons! Edwards et al., arXiv:0708.0768 Secondary Scintillation Primary Scintillation S1 (electroluminescence) S2 6 Single electron sensitivity in S2 1e probability v S1 (photoionisation) 1e ~30 phe Sensitivity: Quantum of ionisation measured in >kg LXe targets Nuclear recoil detection threshold ~0.2 keVnr Calibration: Edwards et al., Astroparticle Phys. 30 (2008) 54 Absolute ionisation yields, electroluminescence, for study of single-electron emission in ZEPLIN-II trigger thresholds, electron lifetime, etc 7 ZEPLIN-III design features { Good light collection for scintillation z Photomultippqliers immersed in the liquid z Slab geometry (35 mm LXe height, 390 mm dia) { Improved discrimination z ‘Open plan’ target, no extraction grids z High field operation (4 kV/cm) z Precision 3D position reconstruction { Low background construction z Copper construction, low background Xe 12 kg LXe 8 ZEPLIN-III: Entrails 9 Boulby Underground Laboratory We’re all in the gutter, but some of us are looking underground… Depth1100m(2.8kmw.e.) 10 Shielding castle { Shielding against rock radioactivity { 30 cm hydrocarbon, 20 cm boxed lead { 105 attenuation for both neutrons and gamma-rays 11 Co-57 daily calibration { S1 calibration defines energy scale for electron (and nuclear) recoils (keVee, keVnr) { Good energy resolution byyp exploiting S1-S2 anti-correlation Ionisation e- contribute either to S1 (recombination) or to S2 (charge extraction) V=5.4%@122keV in FSR, 6.7% with new array (preliminary) VS1 VS2 VE* S2 fraction 12 Nuclear recoil calibration (Am-Be neutrons) 2-20 keVee NR x-yydstbuto distribution biased towards source location WIMP BOX: 2-16 keVee (10-30 keVnr) PVP 5 keVee NR from neutron elastic scatter 13 FSR results – 7 events in search box • 84 days from 27 Feb 2008 –128 kg*days (net) in acceptance region • Predicted neutron elastic scatters in WIMP acceptance box: 1.2±0.6 events •Estimated ER leakage into WIMP acceptance box: 11.6±3.0(stat) events 14 FSR results Lebedenko et al, PRD 80: 052010 (2009) •Most probable WIMP signal P=0 •90% CL limit 3.05 events • SI cross-section limit (W-N) 8.1x10-8 pb/nucleon near 60 GeV/c2 • SD cross-section limit (W-n) 1.0x10-2 pb/neutron near 60 GeV/c2 WIMP mass, GeV/c2 Lebedenko et al, PRL 103: 151302 (2009) 15 FSR results - iDM Schmidt-Hoberg & Winkler, JCAP09(2009)010 Akimov et al., arXiv:1003.5626v2 (ZEPLIN-III) 16 Best discrimination in two-phase xenon •Detector tilted steadily throughout run due to local geology (~1 mrad) • Tilt measurement to 0.06 mrad accuracy with one day’s calibration data •7 events turn to 5 –now with no significant spatial bias •e/n-recoil discrimination improves to 1:7400 in 10-30 keVnr 17 Phase II Calibration { Automated Co-57 source delivery for improved reproducibility { Fibre-coupldled LED system for routine PMT caliblibtiration { Calibrated Am-Be neutron source for Leff new measurement { “phantom” grid above anode for position reconstruction { FWHM of 2 mm in the horizontal plane for Co-57 { Tens of Pm in the vertical direction simulation 18 Phase II Phototube upgrade { PMT gamma-rays limited sensitivity of first run { Custom design for ultra low-background tubes, pin-by-pin compatible { Assembly onto ZEPLIN-III array completed (eventually…) 19 Phase II Phototube upgrade { Factor ~30 improvement in gamma-ray activity per PMT (35-50 mBq/PMT) { Factor ~10 improvement in overall electron-recoil background (depth) (depth) 20 Phase II Anti-coincidence veto { 52-ch (~3S) anti-coincidence veto replaces some hydrocarbon shielding { An important tool for neutron rejection as well as diagnostics { Inner Gd-loaded hydrocarbon surrounded by plastic scintillator modules { Delayed coincidence detection of capture gammas from Gd (and H) J n 21 Phase II Anti-coincidence veto For veto hardware description Akimov et al., Astropart. Phys. (2010) 22 Veto neutron tagging efficiency { Neutrons detected by radiative capture in Gd-loaded plastic { 70% veto efficiency for internal neutrons 23 Veto gamma-ray tagging efficiency { 30% tagging efficiency for low-energy gamma-rays hitting ZEPLIN-III { Useful signal-free sample of dominant background: more robust prediction prompt gammas accidental coincidences 24 Second Science Run Sensitivity { Neutron bk: 0.5 evt/year in 5-50 keVnr, vetoed to 0.15 evt/year { Gamma bk: 14x lower, bett er control of systemati cs with new veto ZEPLIN-III XENON100 WARP 140 LUX350 25 LUX-ZEPLIN – tonne-scale targets { US LUX team and European ZEPLIN team join forces { LZ 1.2 -tonne fiducial xenon at SUSEL (4850 ft level at Homestake, USA) { LZ20 20-tonne xenon at DUSEL (7200 ft level) – WIMP astrophysics? { Coordinated proposals to NSF/DOE/STFC/… Summary { Second science run under way z High duty cycle, operation largely automated z Post-upgrade background exactly as predicted z Anti-coincidence veto fullyyp operational { Nuclear recoil scintillation efficiency in liquid xenon z New measurement soon, with reduced systematics z Especially important for light WIMP scenarios { Sensitivity ~1x10-8 pb/n with one year of running { LUX-ZEPLIN z Very strong collaboration established z Searching full SUSY space for detection and measurement z But plans delayed (need to reward UK bankers first…) 27.