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

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

The solar neighborhood 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! February 1st, 2011 Ray Bunker-UCSB HEP Group

Cosmological Constant 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, Metals (us) 0.01% Visible Baryons 0.5% Dark Baryons 4% Cold Dark Matter 23% Cosmological Constant Dark Energy  73% S.A. Thomas, F. B. Abdalla, and O. Lahav, Phys. Rev. Lett. 105, 031301 (2010). Physics beyond the Standard Model? February 1st, 2011 Ray Bunker-UCSB HEP Group

Production suppressed WIMPsA Dark Matter Candidate Weakly Interacting Massive Particles Massive ↔ Structure Formation Weakly Interacting ↔ Non-observance Relic abundance obtained when annihilation too slow to keep up with expansion Being produced and annihilating (T ≥ MWIMP) Production suppressed (T < MWIMP) WIMP quarks, leptons, photons Freeze out WIMP  1/annihilation A Weak-scale Coincidence? annihilation ~ weak scale yields observed WIMP ~ ¼ ! February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 0 mass bound Chargino mass bound of ~103 GeV/c2  0 mass bound of 4060 GeV/c2 Generally presumes gaugino mass unification February 1st, 2011 Ray Bunker-UCSB HEP Group

Loose Interpretation of 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 0-nucleon cross section (nb) Bottino et al., Phys. Rev. D69, 037302 (2004) 0 mass (GeV/c2) CDMS 2002 Limit 5 keV Threshold EDELWEISS 2002 Upper Limit Loose Interpretation of DAMA Allowed Region Belanger et al., J. High Energy Phys. 03 (2004) 012 0 mass (GeV/c2) 0-nucleon cross section (pb) 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

A low-energy threshold is critical for detecting light WIMPs! 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] x φ [cm-2day-1] x σ [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 But β << 1  Coherent scattering from entire nucleus  ~A4 enhancement Rate ~ (72.61)4 x 5.1x10-9 [kg-1day-1] ~ 0.1 events [kg-1day-1]… much more approachable 100 GeV/c2 WIMP 5 GeV/c2 WIMP Ge Target Si Target σ = 1x10-41 cm2, vescape = 544 km/s Dark Matter Halo Bulge Thick Disk Thin Disk Sun A low-energy threshold is critical for detecting light WIMPs! February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 phonons H ionization Q scintillation L CDMS EDELWEISS CoGeNT IGEX DRIFT XENON LUX ZEPLIN II & III XMASS DAMA/LIBRA ZEPLIN I DEAP/CLEAN NaIAD ROSEBUD, CRESST II CRESST I, PICASSO, COUPP February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Installing the new DAMA/LIBRA detectors in HP Nitrogen atmosphere IMAGE CREDIT: DAMA/LIBRA Collaboration Evidence for a Light WIMP C. Savage et al., JCAP, 0904, 010 (2009); & JCAP, 0909, 036 (2009); & arXiv:1006.0972v2 (2010) 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? R. Bernabei et al., Eur. Phys. J C67, 39 (2010) February 1st, 2011 Ray Bunker-UCSB HEP Group

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. Aalseth et 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) February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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. Aprile et 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. Angloher et al., Astropart. Phys., 18, 43 (2002) Courtesy of M. Schumann February 1st, 2011 Ray Bunker-UCSB HEP Group

 0 CDMS Detector Technology Holes e- Ge or Si Crystal Standard Ionization Measurement Drift Electrons & Holes with -3 to -6 V/cm Electric Field (Applied to Ionization Electrodes) Inner Disk Ionization Electrode ~85% Coverage Ge or Si Crystal 0 Holes e-  Outer Guard Ring Ionization Electrode Phonon Sensors Held at Ground February 1st, 2011 Ray Bunker-UCSB HEP Group

Z-sensitive Ionization & Phonon-mediated CDMS Detector Technology Q inner outer D C A B R sh I bias SQUID array Phonon A feedback V qbias Z-sensitive Ionization & Phonon-mediated ZIP Detector R0 R T T0 Superconducting Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge (QET) phonon sensors Al quasiparticle trap Aluminum Collector Tungsten Transition Edge Sensor (TES) Ge or Si Crystal quasiparticle diffusion phonons Cooper Pair February 1st, 2011 Ray Bunker-UCSB HEP Group

Lines due to decays of internal radioisotopes tilted 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

: reduced ionization collection CDMS Detector Technology Ionization yield Phonon pulse rise time (s) Time since trigger (s) Phonon pulse height (V) Surface events can be misidentified as nuclear recoils Phonon pulse shape and timing is a powerful discriminator Allows for background-free analysis : reduced ionization collection Bulk  Recoil February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) SQUID Readout (phonon signals) FET Readout (Ionization signals) Cold Stages 4 K to 20 mK m m 17 mwe Active Muon Veto Pb Shield Fridge n Copper n n Polyethylene Detectors Inner Pb shield February 1st, 2011 Ray Bunker-UCSB HEP Group

Electron Capture from L-shell Electron Capture from K-shell 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 1.3 keV from 68Ge & 71Ge Decays Electron Capture from L-shell 10.4 keV from 68Ge & 71Ge Decays Electron Capture from K-shell 66.7 keV from 73mGe Decay Beginning of Run 21 End of Run 21 Cf-252 Neutron Calibration Monte Carlo Data Preliminary February 1st, 2011

CDMS Shallow-site Thresholds ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) SQUID Readout (phonon signals) FET Readout (Ionization signals) Cold Stages 4 K to 20 mK 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 ZIP 4 (Si) Total Phonon Energy (keV) Run Number (6V data) ZIP 2 (Ge) Total Phonon Energy (keV) Run Number (3V data) February 1st, 2011

Shallow-site Neutrons Compton γ Electron Recoils CDMS Shallow-site Event Selection Ge Si 1.3 keV Line 32% 0% Zero-charge Events 30-40% Shallow-site Neutrons 6% 2% Compton γ Electron Recoils 10-20% 14C Contamination β’s 40% Others 2-22% 0-18% 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 1080 Candidates 970 Candidates 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 Inner electrode ionization energy (keV) Outer electrode ionization energy (keV) 202 Candidates 130 Candidates 314 Candidates February 1st, 2011 Ray Bunker-UCSB HEP Group

Exclude new parameter space for WIMP masses between 3 and 4 GeV/c2! 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. Akerib et al. (CDMS), Phys. Rev. D82, 122004 (2010) Exclude new parameter space for WIMP masses between 3 and 4 GeV/c2! February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group The CDMS Deep Site February 1st, 2011 Ray Bunker-UCSB HEP Group

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

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 — February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Deep-site Neutron Background Less than one event expectec for CDMS II Limiting background for SuperCDMS… but how soon? February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Fast-neutron Detection High Energy Neutron No Veto, Small Prompt Energy Deposit Veto    Liquid Scintillator Gadolinium Loaded   Capture on Gd, Gammas (spread over 40 μs) PMT Liberated Neutrons PMT Lead Hadronic Shower Veto Veto February 1st, 2011 Ray Bunker-UCSB HEP Group

Fast-neutron Detection Expected Number of sub-10 MeV Secondary Neutrons Simulated 100 MeV Neutrons Incident on Lead Target Detectable Neutron Multiplicity February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group A Fast-neutron Detector February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Detector Installation Electronics Rack Lead Target Source Tubes February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Detector Installation Cheap Labor Water Tanks February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Detector Installation 20” KamLAND Phototubes February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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 February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Neutron Detection Technique Timing is Everything Neutron capture times  microseconds A few 100 Hz of U/Th background  milliseconds February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Neutron Detection Technique More Gamma Like Background U/Th Gamma Rays 252Cf Fission Neutrons Pulse timing Likelihood More Neutron Like Pulse Height Likelihood February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group 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) February 1st, 2011 Ray Bunker-UCSB HEP Group

Ray Bunker-UCSB HEP Group Understanding Energy Scale Event rate (arbitrary units) ~150 MeV Pulse height (V) ~50 MeV Endpoint February 1st, 2011 Ray Bunker-UCSB HEP Group