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Low-threshold Results from the Cryogenic Dark Matter Search Experiment

Low-threshold Results from the Cryogenic Dark Matter Search Experiment. Ray Bunker—CDMS Collaboration WIN`11 Cape Town, South Africa. Outline. Dark Matter and WIMPs Direct Detection Evidence for a light WIMP Direct Indirect The CDMS experiment Detector technology

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Low-threshold Results from the Cryogenic Dark Matter Search Experiment

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  1. Low-threshold Results from the Cryogenic Dark Matter Search Experiment Ray Bunker—CDMS Collaboration WIN`11 Cape Town, South Africa

  2. Outline • Dark Matter and WIMPs • Direct Detection • Evidence for a light WIMP • Direct • Indirect • The CDMS experiment • Detector technology • Shallow-site low-threshold analysis • Deep-site low-energy analysis • Deep-site Neutrons Ray Bunker-UCSB HEP Group

  3. The Dark Matter Problem Milky Way Galactic Rotation Curve • Use interstellar gas to probe galactic • galactic mass distribution • Appears to contradict the R-1/2 • falloff expected from luminous • matter Vcircular (km/s) Y. Sofue, M. Honma and T. Omodaka arXiv:0811.0859v2 Radius, R (kpc) • Large uncertainties, but why should our • galaxy be any different than others? The solar neighborhood at ~8 kpc and ~220 km/s Still the most compelling evidence for the existence of dark matter in the solar neighborhood! Ray Bunker-UCSB HEP Group

  4. Metals (us) 0.01% Visible Baryons 0.5% Dark Baryons 4% Cold Dark Matter 23% Cosmological Constant Dark Energy  73% The Dark Matter Problem Komatsu et al. (WMAP), arXiv:1001.4538 • Concordance of observations of large-scale • structure, supernovae, and the cosmic • microwave background imply: • Only Standard Model candidate is the neutrino, • however… if • then, S.A. Thomas, F. B. Abdalla, and O. Lahav, Phys. Rev. Lett. 105, 031301 (2010). Physics beyond the Standard Model? Ray Bunker-UCSB HEP Group

  5. Production suppressed (T < MWIMP) Being produced and annihilating (T ≥ MWIMP) Freeze out WIMP quarks, leptons, photons WIMPsA Dark Matter Candidate Relic abundance obtained when annihilation too slow to keep up with expansion • Weakly Interacting Massive Particles • Massive ↔ Structure Formation • Weakly Interacting ↔ Non-observance WIMP WIMP  1/annihilation A Weak-scale Coincidence? annihilation~ weak scale yields observed WIMP ~ ¼ ! Ray Bunker-UCSB HEP Group

  6. The Lightest Superpartner • No stable WIMPs in the Standard Model • SUSY extends physics beyond the SM • Lots of new particles very popular among high energy physicists • The LSP is often a WIMP • Such as the neutralino0: • Non-appearance at LEP or Tevatron ↔ Massive (?) • Neutral ↔ Dark • Conserved R-parity ↔ Stable • LEP 0mass bound • Chargino mass bound of ~103 GeV/c2 0 mass bound of 4060 GeV/c2 • Generally presumes gaugino mass unification Ray Bunker-UCSB HEP Group

  7. Light SUSY WIMPs • Relax gaugino mass unification: • The chargino & neutralino masses are basically uncorrelated • The 0 mass can evade the LEP chargino mass bound • Must invoke cosmological constraints for 0 mass bound Bottinoet al., Phys. Rev. D69, 037302 (2004) Loose Interpretation of DAMA Allowed Region CDMS 2002 Limit 5 keV Threshold Belanger et al., J. High Energy Phys. 03 (2004) 012 0-nucleon cross section (nb) • Scanning SUSY parameter space • Belanger et al. find 0 masses as low as 6 GeV/c2 • Lines indicate the sensitivities of the ZEPLIN I (solid), ZEPLIN II (dashed), CDMS (dash-dotted) and EDELWEISS (dotted) experiments • Similarly, Bottino, Donato, Fornengo and Scopel also find 0 masses as low as 6 GeV/c2 • Red points for   CDMmin • Blue points for  < CDMmin EDELWEISS 2002 Upper Limit 0-nucleon cross section (pb) 0 mass (GeV/c2) 0 mass (GeV/c2) Ray Bunker-UCSB HEP Group

  8. Direct Detection • Standard assumption  • Galactic WIMP Halo • WIMP “wind” with ~220 km/s relative • velocity, or β = v/c ~ 7x10-4 • Direct detection attempts to measure: • Erecoil ~ ½ Mnucleus c2 β2 • ~ 10 to 20 keV • Event rate  detector size, •  WIMP flux, & •  cross section • More specifically, sensitivity depends • on detector composition, WIMP • mass, detection threshold, and • halo model Very roughly: Rate = N [atoms] xφ [cm-2day-1] xσ [cm2/atom] N = 8.3x1024 [atoms in a 1 kg Ge detector] φ = 6.1x109 [cm-2day-1] σ = 1x10-43 [cm2/atom] (weak scale cross section) Rate = 5.1x10-9[kg-1day-1]… totally hopeless rate per nucleon Butβ << 1 Coherent scattering from entire nucleus  ~A4enhancement Rate ~ (72.61)4 x 5.1x10-9 [kg-1day-1] ~ 0.1 events [kg-1day-1]… much more approachable σ = 1x10-41 cm2, vescape = 544 km/s Dark Matter Halo Ge Target Si Target Thick Disk Thin Disk 5 GeV/c2 WIMP Sun Bulge 100 GeV/c2 WIMP A low-energy threshold is critical for detecting light WIMPs! Ray Bunker-UCSB HEP Group

  9. Direct Detection • Rate of interactions due to known backgrounds ~103 [kg-1day-1] !!! • With low threshold (~1 keV), the expected rate for a light WIMP (< 10 GeV/c2) is • much larger… ~ 10 [kg-1day-1] • Backgrounds rates increase rapidly at low energies (< 10 keV)… offsetting • higher expected rate for light WIMPs Ray Bunker-UCSB HEP Group

  10. ionization Q L scintillation H phonons Direct Detection • Strategies for overcoming backgrounds: • Passive & active shielding • All Experiments • Minimum ionizing threshold suppression • PICASSO & COUPP • Large detector size, self shielding • DAMA & XENON • Measure 2 signals • CDMS & LUX • Event rate modulation • DAMA & DRIFT • Low threshold • CoGeNT • Pulse shape & timing • CDMS CoGeNT IGEX DRIFT XENON LUX ZEPLIN II & III XMASS CDMS EDELWEISS DAMA/LIBRA ZEPLIN I DEAP/CLEANNaIAD CRESST I, PICASSO, COUPP ROSEBUD, CRESST II Ray Bunker-UCSB HEP Group

  11. Evidence for a Light WIMP Installing the new DAMA/LIBRA detectors in HP Nitrogen atmosphere IMAGE CREDIT: DAMA/LIBRA Collaboration • The DAMA/LIBRA experiment located in the Gran Sasso • Laboratory (Italy): 200 kg of low-activity NaI operated • from September 2003 to September 2009 • Annual modulation in their residual event rate with • correct phase and period… significance of ~9σ • Savage et al. have interpreted their data in terms of spin- • independent WIMP-nucleon interactions… evidence for a • light WIMP? C. Savage et al., JCAP, 0904, 010 (2009); & JCAP, 0909, 036 (2009); & arXiv:1006.0972v2 (2010) Ray Bunker-UCSB HEP Group R. Bernabeiet al., Eur. Phys. J C67, 39 (2010)

  12. Evidence for a Light WIMP • The CoGeNT experiment operates a ~½ kg Ge • diode detector... very low background & • very low threshold • In a short exposure, they observe an excess in • their event rate that has the exponential shape • expected for a light WIMP C.E. Aalsethet al., arXiv:1002.4703v2 • D. Hooper et al. performed a combined analysis of DAMA/LIBRA and CoGeNT data and find a region of consistency that points to a WIMP with: • MWIMP ~ 7.0 GeV/c2 &σWIMP-nucleon~ 2.0x10-40 cm-2 • Hooper et al., Phys. Rev. D82, 123509 (2010) Ray Bunker-UCSB HEP Group

  13. Indirect Evidence for a Light WIMP D. Hooper and L. Goodenough, arXiv:1010.2752v2 D. Hooper and L. Goodenough, arXiv:1010.2752v2 • The FERMI Gamma Ray Space Telescope launched • in 2008 • The Large Area Telescope (LAT) has observed • gamma rays from the galactic center, • 300 MeV to 100 GeV • Dan Hooper & Lisa Goodenough have analyzed • the 1st two years worth of data for a WIMP • annihilation signal • Emission spectrum from 1.25° to 10° is • consistent with π0 decay, inverse Compton • scattering and Bremsstrahlung • Inner 0° to 1.25°, however, shows an excess • Profile is consistent with a cusped halo of • 7-10 GeV/c2 WIMPs, annihilating primarily • into tau pairs D. Hooper and L. Goodenough, arXiv:1010.2752v2 Ray Bunker-UCSB HEP Group

  14. Direct Detection Low-mass WIMP Constraints • Best constraints from the XENON100 experiment... however, low-energy scale controversial: • Red dotted line = constant extrapolation • Red solid line = decreasing extrapolation • E. Aprileet al., Phys. Rev. Lett., 105, 131302 (2010). • The final CDMS II Ge limit is competitive • with 10 keV threshold: • Black solid line • Z. Ahmed et al., Science, 327, 1619 (2010). • Very low-mass limit from the CRESST, • ~½ keV threshold: • Blue dashed line • G. Angloheret al., Astropart. Phys., 18, 43 (2002) Courtesy of M. Schumann Ray Bunker-UCSB HEP Group

  15. 0 CDMS Detector Technology Standard Ionization Measurement Drift Electrons & Holes with -3 to -6 V/cm Electric Field (Applied to Ionization Electrodes) Inner Disk Ionization Electrode ~85% Coverage Holes e- Outer Guard Ring Ionization Electrode Ge or Si Crystal Phonon Sensors Held at Ground  Ray Bunker-UCSB HEP Group

  16. SQUID array Phonon A R sh R feedback quasiparticle trap I Al bias Tungsten Transition Edge Sensor (TES) Aluminum Collector quasiparticle diffusion R Cooper Pair T R0 A D T0 B C Ge or Si Crystal phonons Q outer Q inner V qbias CDMS Detector Technology Z-sensitive Ionization &Phonon-mediated ZIP Detector Superconducting Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge (QET) phonon sensors Ray Bunker-UCSB HEP Group

  17. CDMS Detector Technology • True recoil energy (Erecoil) measured on event-by-event basis by subtracting Luke phonons: • Ionization yield, Y ≡ Q / Erecoil • Excellent separation between electron • recoils and nuclear recoils caused by • neutrons from 252Cf source • Subtracting Luke phonons via average • ionization behavior more reliable for • low-energy nuclear recoils Lines due to decays of internal radioisotopes tilted Electron Recoils Nuclear Recoils Ray Bunker-UCSB HEP Group

  18. : reduced ionization collection Bulk  Recoil CDMS Detector Technology • Surface events can be misidentified as • nuclear recoils • Phonon pulse shape and timing is • a powerful discriminator • Allows for background-free analysis Phonon pulse height (V) Phonon pulse rise time (s) Time since trigger (s) Ionization yield Ray Bunker-UCSB HEP Group

  19. m m FET Readout (Ionization signals) SQUID Readout (phonon signals) n CDMS Shallow-site Run • First tower of CDMS II ZIP detectors operated at shallow Stanford Underground Facility • Total Ge detector mass of ~0.9 kg and total Si mass of ~0.2 kg • “Run 21” WIMP-search data taken between December 2001 • and June 2002, yielding 118 live days of raw exposure • Run 21 split into two periods distinguished by voltage bias used: • 1st half with Ge (Si) operated with 3V (4V) bias voltage (3V data) • 2nd half with all detectors operated with 6V bias voltage (6V data) • Analysis of 3V data with 5 keV recoil energy threshold • published in 2002… Phys. Rev., D66, 122003 (2002) • 6V data previously unpublished 17 mwe Active Muon Veto Cold Stages 4 K to 20 mK Pb Shield Fridge ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) Copper n n Polyethylene Inner Pb shield Detectors Ray Bunker-UCSB HEP Group

  20. CDMS Shallow-site Energy Calibration • Electron-recoil energy scale calibrated with gamma-ray sources (137Cf &60Co) • Ge energy scale confirmed with lines • from decays of internal radioisotopes • Confirmed 11.4 day half-life of 68Ge and • 0.12 ratio of L- to K-shell captures • Si scale more difficult! • Nuclear-recoil energy scale the most important • Calibrated with neutrons from 252Cf source • Ionization yield agrees well with expectation • from Lindhard theory • Ultimately, compare to GEANT simulation: • Ge scale consistent (at low energy) • Corrected Si for ~15% discrepancy 10.4 keV from 68Ge & 71Ge Decays Electron Capture from K-shell 66.7 keV from 73mGe Decay 1.3 keV from 68Ge & 71Ge Decays Electron Capture from L-shell Beginning of Run 21 Cf-252 Neutron Calibration End of Run 21 Monte Carlo Data Preliminary

  21. FET Readout (Ionization signals) SQUID Readout (phonon signals) CDMS Shallow-site Thresholds • ZIP 1 rejected as a low-threshold detector • Hardware trigger efficiency: • Average ionization yield used to estimate recoil energy • Hardware thresholds vary from ~0.7 to 1.8 keV • Software phonon energy threshold • Based on Gaussian width of sub-threshold noise • Events required to exceed 6σ noise width • Software thresholds vary from ~0.6 to 1.6 keV • Ultimate threshold efficiency • Ge thresholds 0.7 to 1.1 keV • Si thresholds 1.5 to 1.9 keV Cold Stages 4 K to 20 mK ZIP 2 (Ge) ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) Total Phonon Energy (keV) Total Phonon Energy (keV) ZIP 4 (Si) Run Number (6V data) Run Number (3V data)

  22. CDMS Shallow-site Event Selection Ge Si • WIMP candidates must pass several data cuts: • Data-quality cuts 99% efficient • Fiducial-volume cut~83% efficient • Single-scatter criterion100% efficient • Muon-veto cut~70-80% efficient • Nuclear-recoil cut~95% efficient • Combined data cuts~50-60% efficient • 1080 candidate events in 72 kg-days of Ge exposure • 970 candidate events in 25 kg-days of Si exposure • Are these really WIMPs?... probably not! • While a low-mass WIMP could be hiding in • these data, we can claim no evidence of a • WIMP signal Raw Spectrum in Blue Corrected for Cut Efficiency in Black Further Corrected for Threshold Efficiency in Orange Average Combined Efficiencies in Orange 90% (statistical) Lower-limit Efficiencies in Blue 1080 Candidates 202 Candidates Outer electrode ionization energy (keV) 970 Candidates 314 Candidates 130 Candidates Inner electrode ionization energy (keV) Ray Bunker-UCSB HEP Group

  23. CDMS Shallow-site Low-threshold Limits Hooper et al. combined: Gray CDMS Shallow-site Ge: Black — CDMS Shallow-site Si: Gray — CoGeNT 2010: Orange --- CRESST Saphire 2002: Blue --- XENON100 Decreasing: Red — XENON100 Constant: Red ···· • Large background uncertainties preclude background subtraction • We use Steve Yellin’s Optimum Interval Method • (specially adapted for high statistics) • Serialize detector intervals to make best • use of lowest-background detectors • Include the effect of finite • energy resolution near threshold • Standard WIMP halo model with • 544 km/s galactic escape velocity • Systematic studies indicate limits • are robust above ~3 GeV/c2 D. Akeribet al. (CDMS), Phys. Rev. D82, 122004 (2010) Exclude new parameter space for WIMP masses between 3 and 4 GeV/c2! Ray Bunker-UCSB HEP Group

  24. The CDMS Deep Site 17 mwe at SUF yielding ~500 Muons per second in the CDMS shielding 5.2x104 m-2y-1 2100 mwe 2100 mwe at Soudan yielding <1 Muon per minute in the CDMS shielding Ray Bunker-UCSB HEP Group

  25. The CDMS Deep Site Ray Bunker-UCSB HEP Group

  26. CDMS Deep-site Low-energy Analysis • Focused low-energy analysis of CDMS WIMP-search data taken at the Soudan Mine • 5 Towers of ZIP detectors (30 total) operated from October 2006 to September 2008 (6 distinct runs) • 8 lowest-threshold Ge detectors • analyzed with 2 keV threshold

  27. CDMS Deep-site Low-energy WIMP Candidates • Optimized nuclear-recoil selection to avoid zero-charge event background • Band thickness due to variations in nuclear-recoil criterion from run to run • Recoil energy estimated from phonon signal & average ionization yield behavior Tower 1-ZIP 5 Ray Bunker-UCSB HEP Group

  28. CDMS Deep-site Low-energy Backgrounds • A factor of ~10 reduction in background levels • Improved estimates of individual background • sources • Comparable detection efficiency for much larger • exposure (~3.5x) • No evidence of a WIMP signal Candidate Spectrum: Black Error Bars Zero-charge Events: Blue Dashed Surface Events: Red + Bulk Compton γ Events: Green Dash-dotted 1.3 keV Line: Pink Dotted Combined Background: Black Solid Average Efficiency Ray Bunker-UCSB HEP Group

  29. CDMS Deep-site Low-energy Limit Z. Ahmed et al. (CDMS), arXiv:1011.2482v1 (submitted to Phys. Rev. Letters) Hooper et al. combined: Gray CDMS Shallow-site Ge: Black — CDMS Shallow-site Si: Gray — CRESST Saphire 2002: Blue --- XENON100 Decreasing: Orange — XENON100 Constant: Orange ···· CDMS Deep-site Ge:Red — Ray Bunker-UCSB HEP Group

  30. CDMS Deep-site Low-energy Spin-dependent Limit CDMS II Ge Deep-site 10 keV Threshold CRESST Saphire 2002 3σ DAMA Allowed Region CDMS II Ge Deep-site 2 keV Threshold XENON10 Ray Bunker-UCSB HEP Group

  31. Deep-site Neutron Background • Less than one event expectec for CDMS II • Limiting background for SuperCDMS… but how soon? Ray Bunker-UCSB HEP Group

  32. High Energy Neutron No Veto, Small Prompt Energy Deposit      Capture on Gd, Gammas (spread over 40 μs)      Hadronic Shower Liberated Neutrons Fast-neutron Detection Veto Liquid Scintillator Gadolinium Loaded PMT PMT Lead Veto Veto Ray Bunker-UCSB HEP Group

  33. Fast-neutron Detection Expected Number of sub-10 MeV Secondary Neutrons Simulated 100 MeV Neutrons Incident on Lead Target Detectable Neutron Multiplicity Ray Bunker-UCSB HEP Group

  34. A Fast-neutron Detector Ray Bunker-UCSB HEP Group

  35. Detector Installation Electronics Rack Lead Target Source Tubes Ray Bunker-UCSB HEP Group

  36. Detector Installation Cheap Labor Water Tanks Ray Bunker-UCSB HEP Group

  37. Detector Installation 20” KamLAND Phototubes Ray Bunker-UCSB HEP Group

  38. Neutron Detection Technique • Water-based neutron detector is challenging! • Small fraction of energy visible as Cerenkov • radiation • Poor energy resolution smears U/Th gammas • into signal region Ray Bunker-UCSB HEP Group

  39. Neutron Detection Technique • Timing is Everything • Neutron capture times  microseconds • A few 100 Hz of U/Th background  milliseconds Ray Bunker-UCSB HEP Group

  40. Neutron Detection Technique More Gamma Like Background U/Th Gamma Rays 252Cf Fission Neutrons Pulse timing Likelihood More Neutron Like Pulse Height Likelihood Ray Bunker-UCSB HEP Group

  41. Understanding Energy Scale Background U/Th Gamma Rays Actual data: shaded red Simulated data: black lines 252Cf Fission Neutrons Pulse height (mV) Event rate (arbitrary units) 60Co ~1 MeV Gamma Rays Pulse height (mV) Ray Bunker-UCSB HEP Group

  42. Understanding Energy Scale Event rate (arbitrary units) ~150 MeV Pulse height (V) ~50 MeV Endpoint Ray Bunker-UCSB HEP Group

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