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SEARCHING FOR WIMPS UNDERGROUND: THE EXPERIMENTAL QUEST

SEARCHING FOR WIMPS UNDERGROUND: THE EXPERIMENTAL QUEST. Henrique Araújo Imperial College London IOP2011 NPPD CONFERENCE 3-7 April 2011, University of Glasgow. Outline. WIMP scattering signal The experimental challenge Recent results Great expectations. What are we looking for?.

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SEARCHING FOR WIMPS UNDERGROUND: THE EXPERIMENTAL QUEST

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  1. SEARCHING FOR WIMPS UNDERGROUND:THE EXPERIMENTAL QUEST Henrique Araújo Imperial College London IOP2011 NPPD CONFERENCE 3-7 April 2011, University of Glasgow

  2. Outline • WIMP scattering signal • The experimental challenge • Recent results • Great expectations H. Araújo

  3. What are we looking for? WIMPs attract most experimental effort A neutralino LSP would make a great WIMP WIMPs should scatter off ordinary nuclei producing measurable nuclear recoils Scalar (SI) and axial-vector (SD) c-N interactions (neutral current exchange) H. Araújo

  4. Low energy nuclear recoils • Elastic scatter off nucleus: • Decreasing, featureless spectrum of low-energy recoils (<~50 keV) • Rate depends on target mass & spin, WIMP mass & spin, DM halo, … • Neutrons are irreducible background • Inelastic scatter off nucleus: • Short-lived, low-lying excited states (easier signature?) • 129Xe(3/2+→1/2+) + g(40 keV), 73Ge(5/2+→9/2+) + g(13 keV) • Neutrons are irreducible background • Inelastic dark matter (iDM): • “particles will scatter at DAMA but not at CDMS” (Smith & Weiner 2001) • Recoil spectrum with threshold (mass splitting, d) • Neutrons are irreducible background H. Araújo

  5. Signal calibration Ge (CDMS-II) Xe (X100) • 100 GeV WIMP on Xe (A=131): • 220 km/s WIMP → ER,max = 40 keV • 1 MeV neutron →ER,max = 30 keV • Neutron elastic scattering populates WIMP acceptance region • Calibration of detection efficiency with Am-Be (a,n), Cf-252 (SF), D-D, D-T sources H. Araújo

  6. Elastic scattering rates Canonical model (‘we’re all in it together’) • Isothermal sphere (no lumps), r∝ r−2 • Local density r0~0.3 GeV/c2/cm3 (~1/pint at 100 GeV) • Maxwellian (gaussian) velocity distribution • Characteristic velocity v0=220 km/s, • Local escape velocity vesc=600 km/s • Earth velocity vE=232 km/s H. Araújo

  7. Elastic scattering rates • Coupling to protons and neutrons more useful than coupling to nucleus • To compare different target materials, indirect searches, LHC results 1. Spin-independent (scalar) interaction • note A2 in enhancement factor • cMSSM-favoured XS within reach of current detectors 2. Spin-dependent (axial-vector) interaction • note J (nuclear spin) instead of A2 enhancement • cMSSM-favoured XS out of reach for the time being… H. Araújo

  8. SI scattering rates for 1 kg targets Probably just around the corner by end 2011 H. Araújo

  9. The experimental 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 kTfiducial mass (self-shielding) • But doing both is hard! Small is better for collecting signal Large is better for background • And there is no trigger… H. Araújo

  10. Backgrounds • Nuclear recoils – same signature • Radioactivity neutrons: (a,n) and SF from U/Thcontamination • Laboratory walls, shields, vessels, components, target material • Neutrons from atmospheric muon spallation • Difficult to shield completely even underground • Recoils from alpha emitters (e.g. Rn-222 and progeny) • Contaminating active target bulk/surfaces, air, etc • Eventually, even coherent neutrino-nucleus scattering! • Electron recoils – discrimination power is limited • Gamma-ray background external to target • K-40, Cs-137, U/Th from walls, shields, vessels, components • Contamination of target bulk and surfaces • U/Th betas and gammas (Pb-214, Bi-214, Pb-210,…) • Cosmogenic (Ge-68, Ge-71,…), anthropogenic (Kr-85, Cs-137,…) H. Araújo

  11. ionisation Q L scintillation H phonons Discrimination Ionisation Detectors Targets: Ge, Si, CS2, CdTe CoGeNT, DRIFT, GENIUS, HDMS, IGEX, NEWAGE Light & Ionisation Detectors Targets: Xe, Ar ArDM, LUX, WARP, XENON, ZEPLIN cold (LN2) Heat & Ionisation Bolometers Targets: Ge,Si CDMS, EDELWEISS cryogenic (<50 mK) Scintillators Targets: NaI, Xe, Ar ANAIS, CLEAN, DAMA, DEAP, KIMS, LIBRA, NAIAD, XMASS, ZEPLIN-I Bolometers Targets: Ge, Si, Al2O3, TeO2 CRESST-I, CUORE, CUORICINO Bubbles & Droplets CF3Br, CF3I, C3F8, C4F10 COUPP, PICASSO, SIMPLE Light & Heat Bolometers Targets: CaWO4, BGO, Al2O3 CRESST, ROSEBUD cryogenic (<50 mK) H. Araújo

  12. Phonons (microcalorimetry) Cryogenic: T0~50 mK Thermal phonon signal is lost with increasing mass: must collect phonons before they thermalise in absorber • Superconducting Transition-Edge Sensor (as in CDMS) • Collect high-frequency (athermal) phonons from particle interaction • Into superconducting Al contacts (threshold 2DAl~ meV) • Quasiparticles from broken Cooper pairs diffuse into a W TES • SQUID readout offers extremely high sensitivity • Channel threshold: 1 keV for Ge & Si nuclear recoils J. Cooley, CDMS Collaboration H. Araújo

  13. Scintillation (photomultipliers) DAMA/LIBRA Collaboration Room temperature, cold or cryogenic NaI, CsI, CaWO4, LXe, LAr: many materials scintillate… Photomultipliers: ancient vacuum tube technology, but no-one has come up with a better alternative yet (and we’re trying…) • Scintillation detectors (as in DAMA) • Best photomultipliers now approaching 50% quantum efficiency • Best NaI(Tl) crystals yield ~90 photons/keV for gamma rays • Typically require coincidence of two photomultipliers (2 phe) • Threshold: 0.3-3 keV for I nuclear recoils • (depending on “channelling” effect) H. Araújo

  14. S2 (electroluminescence) Ionisation(Electroluminescence, TES, HEMT, JFET) Cold: T0~200 K Difficult to measure one electron, but not so hard to measure electroluminescence photons from one electron • Two-phase xenon detectors (as in ZEPLIN) • Strong electric field across liquid-gas xenon target • Collect ionisation from particle track in liquid Xe • Drift up to surface, then emit into vapour phase • Electroluminescence photons detected with photomultipliers • Threshold: 0.2 keV for Xe nuclear recoils 1e Edwards et al., Astroparticle Phys. 30 (2008) 54 H. Araújo

  15. LUX  LUX-ZEPLIN 1.5t Neutrons (5-25 keV) Gammas (5-25 keV) Self-shielding in noble liquids Liquid xenon r=3 g/cm3 Sacrificial volume neutron gamma Fiducial volume S2 S1 S2 S2 H. Araújo S2

  16. LUX  LUX-ZEPLIN 1.5t Neutrons (5-25 keV) Gammas (5-25 keV) Anticoincidence detectoraround WIMP target veto Liquid Xenon make thin! neutron gamma Water cherenkov, passive LXe, bare or loaded scintillator,… H. Araújo

  17. Anticoincidence detectoraround WIMP target Akimov et al, arXiv:1103.0393 • A veto buys you: • Background reduction • Up to order of magnitude for gammas and neutrons • Diagnostic power • Unexpected backgrounds • Radiation environment • Signal-free background sample • Calculation of background expectations without compromising blind analysis Effect of veto efficiency on the discovery power of a rare event search with a single background and no additional discrimination NT is the number of tagged events observed H. Araújo

  18. Recent and future excitements H. Araújo

  19. DAMA/LIBRA: Scintillation GRAN SASSO Target: 250 kg NaI(Tl) 8.9s CL modulation over 13 annual cycles Barnabeiet al, arXiv:1002.1028 (Something is modulated, but what?) H. Araújo

  20. ANAIS: Scintillation CANFRANC Target: aiming for 250 kg NaI(Tl) With 500 kg.years data, the DAMA result could be reproduced if threshold ~2 keVee and background <2 evt/kg/day/keV ANAIS STATUS XXXIX IMFP CANFRANC 10-FEB-2011 Carlos Pobes - Various prototypes developed over last decade - Excessive K-40 contamination in existing crystals - Radio-pure detectors under development - Mass production from end 2011 Anais-0 being tested at old LSC Ready to be installed in new LSC facilities H. Araújo

  21.  S. Henry tomorrow EDELWEISS-IIIonisation & Phonons MODANE Target: 4 kg Ge 384 kg·days from 14 months of operation 5 candidate events above 20 keV estimated background is <3.0 events sSI<4.4×10−8pb (90% CL) at 85 GeV E. Armengaudet al, arXiv:1103:4070v2 H. Araújo

  22. CRESST: Scintillation & Phonons GRAN SASSO Target: 3 kg CaWO4 Observed 57 events (yes, fifty seven!) in 730 kg*days in oxygen band Background prediction 35.6 events (of which 17.3 from neutrons, measured from only 3 multiple scatters) J. Schmaler (German Physical Society meeting, 30 Mar 2011) H. Araújo

  23. Light WIMPs: ‘excesses’ at low energies DAMA CoGeNT CRESST A ‘glimmer’ or a ‘flicker’? Phys. Rev. 26, 71–85 (1925) CMSSM Buchmueller et al CMSSM Trotta et al Aalseth et al, arXiv:1002:4703v2) H. Araújo

  24. What would ZEPLIN-III make of it? You cannot be serious! 13 GeV WIMP AT sSI=3x10-5pb Z3 FIRST RUN OBSERVATION: 7 events near top of acceptance region in 2-16 keVee Recoil spectrum in xenon 140 kg*days in FSR signal box 30 events >2 keVee! H. Araújo

  25. COUPP: Bubble chamber SNOLAB Target: 3.5 kg CF3I E. Behnke et al, PRL 106, 021303 (2011) Run at shallow site (Fermilab): 3 candidate events were observed in 28.1 kg.days, consistent with neutron background. Electron recoils do not nucleate bubbles Background from neutrons and alphas Ultrasound emission provides powerful discrimination between alphas and nuclear recoils (as demonstrated by PICASSO) H. Araújo

  26.  wed pm parallel session ZEPLIN-III: Scintillation & Ionisation BOULBY Target: 12 kg LXe, 6.5 kg fiducial >280 days continuous operation Result from ~2,000 kg*days soon Sensitivity 1-2x10-8pb CMSSM Buchmueller et al CMSSM Trotta et al H. Araújo

  27. XENON100: PLR analysis Target: 62 kg liquid xenon, 30 kg fiducial E. Aprile, XIV Int. Workshop on Neutrino Telescopes, Venice, 16 Mar 2011 H. Araújo

  28. XENON100: “result in weeks” Target: 62 kg liquid xenon, 30 kg fiducial E. Aprile, XIV Int. Workshop on Neutrino Telescopes, Venice, 16 Mar 2011 “ ‘unblinding procedure’ in final stage of internal review ‘Blind’ analysis of ~10x more data near completion Results expected within weeks. Non negligible discovery potential” H. Araújo

  29. The future GEN-1 ANAIS ARDM COGENT COUPP CRESST DAMA DARKSIDE DM-TPC DRIFT EDELWEISS KIMS LUX350 MiniCLEAN NEWAGE PICASSO SCDMS WARP XENON100 XMASS ZEPLIN-III … GEN-2 DARKSIDE DEAP-3600 EURECA LZS PICASSO-II SCDMS XENON1t XMASS-II GEN-3 COUPP GEODM LZ20 MAX CLEAN H. Araújo

  30. Conclusions Dark matter is one of the hottest topics in science today The field of underground WIMP searches is very vibrant, attracting strong investment worldwide (mustn’t grumble…) Direct, indirect and accelerator searches are finally converging in sensitivity for neutralino-proton interactions An exciting 2011: new results expected from Gen-1 targets and significant design/construction activity at tonne scale H. Araújo

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