Dark Matter Detection with Cryogenic Detectors Dan Bauer, Fermilab + Gilles Gerbier, CEA Saclay The Physics - Identifying Dark Matter particles Direct Detection and Backgrounds Why use cryogenic techniques? Status and results from the experiments Cryogenic Dark Matter Search (CDMS) EDELWEISS CRESST Rosebud Future Prospects TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
The Physics of Dark Matter • Cold dark matter makes up nearly 1/4 of the mass/energy of the universe! • Particle candidates for CDM – WIMPs (GeV-TeV masses) • SUSY neutralinos • Kaluza-Klein excitations – Axions (10 -3 -> 10 -6 eV masses) • Dark matter responsible for galaxy formation (including ours) – We are moving through a dark matter halo • Standard halo assumptions Maxwell-Boltzmann velocity distribution V 0 = 230 km/s, v esc = 650 km/s, � = 0.3 GeV / cm 3 TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
WIMP signal characteristics • Scattering off nuclei courtesy of Gaitskell • A 2 dependence – coherence loss – relative rates • M W relative to M N – large M W - lose mass sensitivity – if ~100 GeV • Present limits on rate • Following a detection (!), many cross checks possible Vary M W for M N =73 from Jungman et al. – A 2 (or J , if SD coupling) – WIMP mass if not too heavy • different targets • accelerator measurements dR/dE R – galactic origin • annual recoil energy, E R (keV) • diurnal/directional - WIMP astronomy TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
Backgrounds: cosmic rays and natural radioactivity WIMP scatters (< 1 evts /10 kg/ day) (< 1 evts /10 kg/ day) swamped by backgrounds ( > 10 ( > 10 6 evts/kg-d) 6 evts/kg-d) Radioactive Nuclides in rock, Cosmic Rays surroundings 238 U, 232 Th chains, 40 K Fast muons Fast muons Radioactive Airborne Radioactive Nuclides Nuclides Radioactivity 222 Rn in detector, shield Slow muons in atmosphere (especially 222 Rn Shield daughters, including contaminants 210Pb t1/2=22 years) Neutron Electrons Electrons ( � , n) capture Muon capture Photo fission Spontaneous fission Gammas Gammas ( � , n) Neutrons Neutrons courtesy of S. Kamat TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
Minimizing backgrounds • Critical aspect of any rare event search • Purity of materials – Copper, germanium, xenon, neon among the cleanest with no naturally occurring long-lived isotopes – Ancient Lead, if free of Pb-210 (T 1/2 = 22 years) • Shielding – External U/Th/K backgrounds bkg free: ~t • Radon mitigation log(sensitivity) • Material handling and assaying bkg: ~t 1/2 systematics – surface preparation – cosmogenic activation • Underground siting and active veto log(exposure) – Avoid cosmic-induced neutrons • Detector-based discrimination TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
The Key to Direct Detection of WIMPS Detecting Low-Energy Nuclear Recoils TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
Nuclear-Recoil Discrimination • Nuclear recoils vs. electron recoils Division of energy Background � WIMP Timing E ionization � Stopping power l a � CDMS II, n g i S EDELWEISS I HPGe expts 10% energy E phonons CDMS II, Ionization EDELWEISS II +timing Picasso, ZEPLIN II/III/Max, Target Simple, Coupp Phonons/heat XENON, LUX, (superheated) WARP, ArDM 100% energy slowest cryogenics Background Light DAMA/LIBRA 1% energy CRESST II E light ZEPLIN-I, l KIMS a fastest n g i S no surface effects DEAP, CLEAN, E phonons WIMP XMASS +timing TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
WIMP-detection Experiments Worldwide SNOLAB Soudan Picasso DUSEL CDMS II DEAP LUX SuperCDMS CLEAN XMASS SIGN FNAL IGEX KIMS COUPP SUF CDMS I LiF Boulby Elegant V&VI NaIAD Gran Sasso ZEPLIN I/II/III DAMA/LIBRA LSM DRIFT 1/2 CRESST I/II EDELWEISS I/II ORPHEUS Genius TF Running CanFranc CUORICINO IGEX XENON ROSEBUD WArP Cryogenic (<77K) ANAIS Experiments ArDM TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
Removing Muon-induced Neutron Background • Neutrons from cosmic rays are CDMS I - Stanford irreducible background • At SUF � 17 mwe CDMS II - Soudan Log 10 (Muon Flux) (m -2 s -1 ) � 0.5 n/kg-d CRESST II • At Soudan EDELWEISS II � 2090 mwe � 0.5 n/10kg-y SuperCDMS • At SNOLab � 6060 mwe � 0.2 n/ton-y Depth (meters water equivalent) TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS - A typical cryogenic experiment Detectors Ge and Si crystals, 6/tower Electronics/ x5 Measure ionization and phonons DAQ Record signals from detectors Cryogenics and veto; Maintain form trigger detectors at 50 mK Shielding Layered shielding (Cu, Pb, polyethylene) reduces radioactive backgrounds and active scintillator veto is >99.9% efficient against cosmic rays. TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS Cryogenics: How to get really cold! Dilution Refrigerator (< 50 mK) Cryocooler (77K and 4K) Removes heat load from signal cables. Icebox (Detector Cold Volume) TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS: Cryogenic “ZIP” detectors Superconducting films that detect minute amounts of heat Transition Edge Sensor sensitive to fast athermal phonons ~ 10mK 1 � m tungsten 4 aluminum fins R TES ( � ) 3 2 1 T (mK) T c ~ 80mK Ionization measurement TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS Techniques for Recoil Discrimination Detectors with readout of both charge and phonon signals • Charge/phonon AND phonon timing different for nuclear and electron recoils; event by event discrimination! • Measured background rejection still improving! 99.9998% for � ’s, 99.79% for � ’s • Clean nuclear recoil selection with ~ 50% efficiency Can tune between signal efficiency and background rejection Tower of 6 ZIPs gammas Tower 1 neutrons gammas 4 Ge betas 2 Si betas Tower 2 2 Ge neutrons 4 Si TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS - Blind analysis to minimize bias • Cuts set on calibration data and non-masked FET WIMP-search data T2 T1 – timing parameter SQUID – ionization yield – problem detectors/channels Calibration data in Detector T2Z3 (Ge) 133 Ba gammas 133 Ba surface betas Tc Gradient noisy “bad” region 14 C contam. 252 Cf neutrons = Ge = Si TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS Soudan Combined Limits 90% CL upper limits assuming • Upper limits on the standard halo, A 2 scaling (Spin. Ind.) Cross section [cm 2 ] (normalized to nucleon) WIMP- nucleon cross DAMA Na ann. mod. section are 1.7 � 10 -43 (Gondolo/Gelmini) cm 2 for a WIMP with mass of 60 GeV DAMA 7-year NaI, Riv. Nuovo Cim. 26N1,2003 (astro-ph/0307403) • Excludes regions of SUSY parameter space under some frameworks � Bottino et al. 2004 in magenta (relax GUT Unif.) � Ellis et al. 2005 (CMSSM) in green WIMP Mass [GeV/c 2 ] 2-tower and combined (53 kg-d): PRL 96 , 011302 (2006) 1-tower (19 kg-d): PRL 93 , 211301 (2004); PRD 72 , 052009 (2005) TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
CDMS - Data run with 5 towers October 2006 - July 2007 - July 2008 • Vital statistics – Base temperature for ~ 9 months – 5 months of high-efficiency data taking ( 430 kg-days Ge ) • 107.4 live days for WIMP search (2.7 million events) • 36 (0.76) million gamma (neutron) calibration events • 4 TB of data • Blind analysis underway – Cuts set using calibration data – Expect to open nuclear recoil region November 2007 – Sensitivity should be x5 better than previous (3 x 10 -8 pb for M W ~ 60 GeV) • July 2007-July 2008 – Aim for another x3 improvement in sensitivity (~1300 kg-d) • Approaching 10 -8 pb or perhaps we might start to see a WIMP signal – May start to run into backgrounds at Soudan • Beta backgrounds on some detectors, Neutrons from cosmic rays – If background-free, run 5 towers through 2008 • Install first SuperCDMS detectors when ready TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
The Reach of CDMS at Soudan DAMA 1 1 - - N N I I L L P P E E Z Z s s s s i i e e w w l l e e d d E E SUSY Models t t n n e e r r r r u u C C - - I I I I S S M M D D C C CDMS II -projected CDMS II -projected TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
What do we learn if we see a signal? • Current 90% C. L. limit corresponds to < 1 evt per 8 kg-d for Ge actual • Most favorable of linear signal collider SUSY models (LCC2) predicts ~5 events in CDMS II at Soudan! • WIMP mass & cross section would be SuperCDMS 25 kg will be ideal for determined as shown and exploring such a WIMP signal on the same time scale as LHC! SI vs SD determined from different targets TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
Next for CDMS: SuperCDMS 25 kg • Proposed 25-kg experiment based on updated 42 x 600-g Ge ZIPs – 120x beyond current limits – 15x beyond CDMS-II goal – Approved for space at SNOLAB – Next step towards ton-scale goal • Detector fabrication and characterization underway nuc. rec. 1”-thick 0.6-kg: 3x fiducial mass per s.a. surface TAUP 2007 - September 11, 2007 Dan Bauer - Fermilab
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