Recent Results & Current Status of C RYOGENIC D ARK M ATTER S EARCH

1. Recent Results & Current Status of CRYOGENICDARKMATTERSEARCH Rupak Mahapatra Univ. of California Santa Barbara APS-DPF 2006+ JPS 2006, Hawaai

2. What Nature has to Offer  0      0                              Background Rejections ‘R’ Us Action Plan: Reduce and Reject …

3. Outline • Motivation & Candidates: Astro + Particle • Detection Principles: Signal & Background • CDMS Background Reduction & Rejection • Results from 1 and 2 Tower Runs • 5-Tower CDMS Run and Current Status

4. m M bad fit to data Use light as a guide for mass Expect v2a 1/r Motivation: Galactic Rotation Curves Is there mass where there is no light? …Dark Matter

5. Metals (us) 0.01% Visible Baryons 0.5% Dark Baryons 4% Cold Dark Matter (WIMPs?) 23% Cosmological Constant Dark Energy  73% Standard Model? In thermal equilibrium after Big Bang. Non-relativistic. A New Order Has Been Declared….

6. Current abundance is related to annihilation cross section to our matter Standard Model Weak Scale Galactic Astrophysics Big Bang 0 0 0 1/ann q, l, Supersymmetry ann ~ weak gives0 = ¼ observed 0 SUSY restored at Weak Scale gives rise to LSP (0) with weak interaction with matter Clue or Coincidence?

7. v/c = 0.7 10-3 Design a Particle and an Experiment 0 v/c = 0.7 10-3 Neutral: 1) cool particles neutral – , n, , K0, Z0, H0… We use Germanium, A=73, mc2=72 GeV; others: Si, S, I, Xe, W ER½ mGec2 2  ½ 72 GeV  ½  10-6  20 keV  x-ray energy ! Easy! Massive: 1) Mc2100 GeV hinted at by accelerator data `Weak Scale’

8. Catalog of Recoil Experiments Rick Gaitskell

9. ER10’s keV Holes Electrode Implants e-  E 0 Traditional Ionization Detector 1 cm Germanium 7.6 cm ¼ kg What rate? (in, say, 1kg) Backgrounds? ….…. Gamma rays, neutrons, surface beta-decay

10. Rate governed by scattering cross section 

11. Donald H. Perkins, 1987 What is the weak interaction cross section?

12. Our Hunting Ground Weakly Interacting Massive Particle Experiment CDMS (shallow) DAMA (old!) Theory SUSY, various constraints including Big Bang Gaitskell/Mandic

13. 0 0 Indistinguishable Density of States: Coherence, density of states enormous bonus! Scattering off a proton…. …. Hopeless! Acceptable Rate. But, what about the Background rate….

14. Rate of Main Background Rate about 103 / (kg-day) !!! 10000X bigger than expected signal Strategies: DAMA… huge target mass, look for astrophysical modulation CDMS… small target mass, distinguish electron from nucl. recoil

15. Direct Detection: Signal and Main Background Background Electron Recoils Er v/c  0.3 Sparse Energy Deposition  Signal Nucleus Recoils Er v/c  710-4 dense energy deposition efficiency low distinct energy scale 0 (calibrate: neutron) Differences the Basis of Discrimination

16. ionization Q L scintillation H phonons Typical Discrimination Technique: Detect More than One Signal IGEX, DRIFTI, II CDMS, EDELWEISS ZEPLIN II, III, MAX, XMAS, XENON CRESST I, PICASSO, COUPP NAIAD, ZEPLIN I, DAMA CRESST II, ROSEBUD

17. 0 Nuclear Recoil bad at making Ionization Holes e- Germanium more ionization! Need a second, `fair’ measure of deposited energy… phonons!  Both deposit, say, 20 keV

18. CDMS Technique: Phonons v. Ionization Nuclear Recoils (neutron source) Electron Recoils ( source) Phonon Energy: True Energy. No Loss Yield = Ionization Energy/Phonon Energy. Extremely Powerful Discriminant

19. CDMS Detectors: ZIP `Phonon sensor (4)’ (TES) Ionization Electrodes (2) x-y-z imaging: from timing, sharing Z-coordinate, Ionization, Phonons ZIP Operate at 0.050 Kelvin

20. quasiparticle trap Al W Transition-Edge Sensor (TES) Al Collector quasiparticle diffusion ~ 10mK Cooper Pair RTES () 4 3 Ge or Si phonons 2 1 normal T (mK) Tc ~ 80mK superconducting The Phonon Sensor

21. Excellent Energy, X-Y Position Reconstruction Am241 : g 14, 18, 20, 26, 60 kev Cd109 + Al foil : g 22 kev Detector Calibration at Berkeley A D Cd109 : g 22 kev i.c. electr 63, 84 KeV B C

22. Excellent Rejection of Primary () Background Yield = Ionization/Phonon Most effective Particle ID Neutrons cause nuclear recoils too! Another background… Phonon

23. Background Neutrons from Cosmic Ray Muons Limited our earlier Stanford results…moved to a deep mine

24. 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 200 Hz muons in 4 m2 shield Stanford Underground Site Oroville (USA) 1 per min Soudan (USA) Kamioka (Japan) Boulby (UK) Log10(Muon Flux) (m-2s-1) Gran Sasso (Italy) Frejus (France) Baksan (Russia) Mont Blanc (France) Sudbury (Canada) Kolar (India) 0 2000 4000 6000 8000 10000 Depth (meters water equivalent) Why Soudan @ -40o

25. CDMS Outside In µ-metal (with copper inside) Ancient lead Phonon Sensors 23 14 41 cm @ 40 mK!! Two `towers’ Polyethylene Low Activity Lead Surround detectors with active muon veto • To further reduce neutron background and electromagnetic background: Use passive shielding • Lead and Copper for photons • Polyethylene for low-energy neutrons

26. CDMS Veto System  • 2” Thick Scintillators • ~ 100% Efficient detection for through going muons • ~90% Efficient detection external neutrons, due to associated hadronic showers • Multiplicity trigger implemented to collect interesting muon events  Ray Bunker, Joel Sander

27. FET cards FET cards SQUID cards SQUID cards 14C 14C 14C CDMS Tower of Detectors • Both Ge and Si for neutron background measurement: Si has higher  with N, than Ge • Possible WIMP mass meas, if we see a signal • Achieved much better sensitivity with Si than Ge for low mass WIMPs Each tower holds 6 ZIPs 4 K 0.6 K 0.06 K 0.02 K ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) worse s

28. Radioactive Source CDMS Geometry Characterize detector response to signal and background using neutron and gamma source, respectively Extensive calibration data throughout run Comparison with MC Veto Scintillator Polyethylene Lead Cables Cryo Detectors

29.  Calibration (133Barium) (e recoils) Phonons Ionization Energy, KeV Laura Baudis

30. Energy Calibration Ionization Phonons 356,  =2.5 keV 275 keV  =8 keV 303 384 • Phonon energy resolution worse than charge at high energy due to incomplete digitization of the full pulse Walter Ogburn

31. n Calib. (252Californium) (nuclear recoils) Reconstructed recoil energy, KeV Sharmila Kamat

32. Revisiting the Most Powerful Particle ID Yield = Ionization/Phonon Most effective Particle ID YIELD Rejects 99.9 % background However, still not enough! Phonon

33. What does real data look like? Yield rejects most Bkg ~ 1 M events Few events in or near signal region. Fundamentally different background. Not tail of  distribution.Dangerous . Just like signal

34. : reduced ionization collection Bulk Recoil Z Why is  Dangerous? • Electrons gets absorbed in the first few microns • Ionization collection inefficient for surface events • Yield = Ionization/Phonon => Yield low for surface events () • Doesn’t completely reject . Need some extra handle •  background ultimately limits sensitivity of many DM experiments Name of the Game is  Reduction and Rejection

35. Calibration: ,  and Neutron • 133Ba  calibration: Used for position and energy calibration •  from 133Ba  :Compton scattered e- • 252Cf neutrons : Signal 20x our WIMP-search background Ionization Yield Yield not enough to cut all  background Need extra rejection handle to reject these s Use Pulse Timing to advantage! Recoil Energy (keV)

36. Phonon Timing  Pulse Faster  10-40% A D  B C (phonon start time) Timing quantities used to suppress external electrons Ionization Pulse gives start time

37. Mean has most of the info… Surface Event() Nuclear Recoils Rejecting  with Timing Information

38. Improved  Rejection: 2 Formalism • Better combining of discriminators • Define 2 hypothesis for signal (neutron) and background () from calibration data • Determine how far a particular event is from the signal (rn) and from background (rb) hypothesis • Define cut to delineate with desired optimum Rupak Mahapatra & Joel Sander

39. Surface events from calibration source neutrons from calibration source Resulting Improvement Rupak Mahapatra & Joel Sander

40. Define all cuts from Calibration Data only 252Cf calibration (N) defines signal region 133Ba calibration ( ) defines  as well as  Half of 133Ba calibration is blinded! Once all cuts are defined, Blinded 133Ba data is used for estimating efficiencies and  leakage Signal region blinded Use data side-bands after full analysis done with calibration data and cuts are frozen Estimate  leakage from side bands and compare with calibration  estimate Understand systematics Calculate expected sensitivity Un-blind. Apply  timing cut Count remaining events. These are candidates CDMS Blind Analysis Technique Calibration Data WIMP-search Data

41. WIMP-search Data Blinding

42. Overall Efficiencies

43. Improved Si Efficiency due to 2 Formalism Rupak Mahapatra & Joel Sander. Jeff Filipini Improvement in low E regime tremendously improves low mass WIMP sensitivity

44. 1 candidate (barely) 15 sig. region 1 near-miss Surface Electrons Unblind: Before/After Timing Cut • ESTIMATE: 0.4  0.2 (sys.)  0.2 (stat.) electron recoils • 0.06 recoils from neutrons expected Ionization Yield Recoil Energy (keV) Z2/Z3/Z5/Z9/Z11

45. Small Circles: prior to surface rejection Blue circles: passing surface rejection Star: one candidate Expected Background(7-100 keV recoil energy) Beta 0.4±0.2±0.2 for Ge and 1.2±0.6±0.2 for Si Neutron 0.06 for Ge and 0.05 for Si

46. Silicon: low mass New Limits (Spin Independent) 90% CL About twice more sensitive than 1-tower

47. 0 Neutralino Spin-Dependent Interaction Z0 Axial vector interaction gives spin-dependent scattering… neutron or proton

48. New Limits (Spin Dependent) CRESST-I Picasso DAMA Zeplin-I NAIAD Super-K Solar Majorana  Majorana  CDMS Has SD Sensitivity Too !!! World-Best for 0 -neutron coupling unpaired neutron 8% 73Ge 5% 29Si proton neutron Si Ge (2 nuclear models) Si Ge (2 nuclear models) Jeff Filipini

49. Two Papers Published This Year

50. The Near Future: 5 Towers run for 2 years in Soudan Installed 3 additional towers in November 04 • Improvements • Cryogenics, backgrounds, DAQ • Currently commissioning • 30 detectors in 5 towers of 6 • 4.75 kg of Ge, 1.1 kg of Si to run through 2006 • Improve sensitivity x10