Transcript Direct Detection of Dark Matter: Status and Perspectives Bruno Serfass – March 2006
Direct Detection of Dark Matter: Status and Perspectives
Bruno Serfass
University of California, Berkeley (CDMS experiment) Rencontres de Moriond – March 2006
Evidence for Dark Matter
Large Scale Structure CMB Fritz Zwicky observed in 1933 anomalous large velocity dispersion in distant galaxy clusters Lensing Since that time many more observations have been made, at different scale, supporting the presence of Dark Matter in the universe.
Rotation curves of galaxies SN Ia Galaxy clusters
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Dark Matter Candidates
Baryonic Dark Matter
• • •
Dust, Gas, Stars MACHOs (< 20% of halo) BBM, light element ratio observations: Ω B = 0.05 ± 0.005
All evidence agrees that baryonic dark matter exists but does not constitute much of the total dark matter
• •
Hot Dark Matter Upper limit from CMB, tritium decay and neutrino oscillations
< 0.0155
Massive neutrinos exist, but not enough mass to explain dark matter Hot dark matter cannot produce observed large scale structure of universe
Cold Dark Matter (rest of this talk)
•
Two well-motivated candidates from particle theory: WIMPs (Weakly Interacting Massive Particles), Axions Theory suggests that either could naturally explain measured matter density
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Weakly Interacting Massive Particles (WIMPs)
Cosmology provides a class of long lived or stable particles at the EW scale left over from Big Bang: If such particles produced in thermal equilibrium in early universe,
Freeze out
when annihilation too slow to keep up with expansion Leaves a relic abundance:
c
h 2
10 -27 cm 3 s -1
ann v
fr For
c
~ 0.3:
• •
M ~ 10-1000 GeV
A ~ electroweak
Independently, supersymmetry theories predict a stable s-particle state whose properties are very similar to the hypothetical WIMPs
Note: not only supersymmetry (extra dimesions Kaluza-Kein excitation, etc.) 4
Searching for Dark Matter
Indirect detection:
•
Look for products of annihilations in the sun or earth (AMANDA, IceCube, Super-K, EGRET, etc.)
•
Make dark matter in accelerators and detect products of interactions (LHC), however
• •
not sensitive to high mass still need to show that particle could make DM (stable particle)
Direct detection:
•
Measure them when they elastically scatter off nuclei in target - Suppress background (go deep underground, shielding..) - Low energy threshold - Large target mass needed
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Direct Detection of WIMPs
Detect WIMPS via elastic scattering on nuclei in targets (nuclear recoils) Energy spectrum & rate depend on target nucleus masses and WIMP distribution in Dark Matter halo: Standard assumptions:
Isothermal and spherical Maxwell- Boltzmann velocity distribution
= 0.3 GeV / cm 3 WIMP detector Measure recoil energy
• •
Energy spectrum of recoils ~ exponential with
n
c
and
) is of the order of a fraction of 1 event /kg/day E recoil Direct detection experiments need low threshold, low background, and high target mass: very challenging!
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Experimental Signatures
Nuclear Recoils : WIMPs produce nuclear recoils, while most of the backgrounds produce electron recoils
• • •
Electron produce more ionization (scintillation) than a nuclear recoil of equal energy Example: ionization Yield = E ionization /E phonon ≈ 0.3 for Ge nuclear recoils However, neutrons also produce nuclear recoils Ionization Liquid Xe (XENON, ZEPLIN II, III, IV) NaI, Xe (DAMA, ZEPLIN I) Ge, Si (CDMS, Edelweiss) CaWO 4 (CRESST II) Al 2 O 3 , LiF
E
phonons Background Signal
E
phonons
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Experimental Signatures
Nuclear Recoils: WIMPs produce nuclear recoils, while most of the backgrounds produce electron recoils
Annual modulation : variation of WIMPs flux with time of year (annual)
• •
Requires long exposure and large mass to measure small effect (~5%) Experiments: DAMA, KIMS, etc.
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Experimental Signatures
Nuclear Recoils: WIMPs produce nuclear recoils, while most of the backgrounds produce electron recoils
Annual modulation: variation of WIMPs flux with time of year (annual)
Directionality: diurnal modulation following the earth rotation on its axis WIMP Wind 42 o 12:00h
•
Difficult measurement: typical recoil range is of order of 20 nm in a crystal (for a 20 keV recoil in Ge) and 20
m
m in gas (30 keV in Kr)
•
Experiments at R&D stage (DRIFT) 0:00h
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Experimental Signatures
Nuclear Recoils: WIMPs produce nuclear recoils, while most of the backgrounds produce electron recoils
Annual modulation: variation of WIMPs flux with time of year (annual)
Directionality: WIMPs produce nuclear recoils, while most of the backgrounds produce electron recoils
No multiple interactions: year; neutrons: order of cm mean free path of a WIMP: order of a light-
can be used to identify neutrons which also produce nuclear recoils
Recoil energy spectrum shape: background exponential, rather similar to
Uniform rate throughout the detector: WIMPs interactions must be spread evenly throughout the detector
if detector significantly larger than mean free path of high-energy photons or neutrons, the interactions will occur mostly near the surface
Consistency between targets of different nuclei signal clearly identified : essential once first
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WIMPs Direct Detection Strategies
2 main fundamental strategies (in practice combined): 1) Statistical method: - Statistically estimate the background Annual modulation , multiple scattering, pulse shape discr.
- Large mass, simple detectors e.g. NaI (DAMA)
sensitivity
M
=Mass
T
=Time 1 MT
2) Event by event discrimination - Active rejection with the best possible discrimination - For example, nuclear vs electron recoils - More sophisticated technology
sensitivity 1 MT 11
Backgrounds for Direct Detection Experiments
m m
Put experiment underground so no cosmic-ray nuclei reach it; very few muons (and hence fast neutrons) if deep enough
Surround detectors with active muon veto Rock
n
detector U/Th/K/Rn
,n Rock U/Th/K/Rn Lead, polyethylene
3 2 1 0 -1 -2 -3 -4 Oroville (USA) Soudan (USA): CDMS Kamioka (Japan) Boulby (UK) : ZEPLIN Gran Sasso (Italy) : DAMA, CRESST Frejus (France) : EDELWEISS -5 0 2000 4000 6000 8000 10000 Depth (meters water equivalent)
Veto (active)
Use clean, low-radioactivity (= screened) materials
Use passive shielding - Pb shielding to reduce EM backgrounds from radioactivity - Polyethylene contains hydrogen needed to moderate neutrons from radioactivity
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WIMP-detection Experiments Worldwide
Funding scale ~$10M/experiment Collaborations: 10-50 physicists/experiment
CDMS II
(Superheated droplets)
Picasso CDMS I
(Cryogenic)
IGEX
(HPGe) (Liq. Xenon)
KIMS XMASS
CsI Boulby NaIAD IGEX
(NaI)
ZEPLIN I/II/III/MAX DRIFT 1/2 ANAIS
(TPC)
CanFranc
(HPGe)
ROSEBUD
(NaI) (Liq. Xenon) (Cryogenic) (Superheated superconducting granules)
ORPHEUS EDELWEISS I/II Majorana
LiF Elegant V&VI
Gran Sasso DAMA/LIBR CRESST I/II HDMS Genius Xenon CUORE
(NaI) (Cryogenic) (HPGe) (Ton of “bare” HPGe) (Liq. Xenon) (TeO 2 )
Next talk 13
DAMA NaI Experiment
Very elegant experimental setup place >1996 in
Low Activity NaI scintillator 9
9.7 kg NaI crystals, each viewed by 2 PMTs - known technology - large mass - spin dependent interaction
Located at Gran Sasso Underground Lab (3.8 kmwe) + Photon and Neutron shielding
Two modes of background discrimination - Pulse shape (not used) Annual modulation : ~2% modulation amplitude NaI NaI NaI NaI Copper Lead Polyethelene Glove box for calibration sources
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DAMA Search for annual modulation
Not distinguish between WIMP signal and Background directly
From the amplitude of the modulation, we can calculate the underlying WIMP interaction rate
105 125 103 100 75
WIMP Signal
50 25
Background
0 -0.5
Dec
-0.1
0.3
June
0.7
Dec
1.1
1.5
June
101 ±2% 99 97 95 -0.5
Dec
-0.1
0.3
June
0.7
Dec
1.1
June
1.5
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DAMA 7-year Annual Modulation Results
Source DAMA Riv. Nuovo Cim 26N1 (2003) 1-73
6.3
annual modulation is observed in the rate BUT, the modulation is only a 5% effect and is all in the lowest energy bin.
What’s next?
DAMA
LIBRA , a 250-kg NaI experiment has been operated since 2003 DAMA discovery claim Conventional halo Scalar coupling
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KIMS - Similar to DAMA but with CsI
Located at 700 mwe underground in Korea
Challenge: internal background from 137 Cs contamination is most problematic.
Can be reduce by using purified water in processing
Recent result is based on 1 crystal with 6.6 kg; Currently running 4 crystals with mass 8.66 kg each
Plan to start taking 100 kg data this summer Test DAMA data with similar crystal detector containing Iodine.
should be helpful to confirm or deny claimed signal
DAMA
Phys. Lett. B 633 (2006) 201
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Cryogenic detectors
Principle: phonon mediated detectors Goals:
Sensitivity down to low energy. Phonons measure the full recoil energy
Active rejection of background (event by event discrimination): recognition of nuclear recoil combine - with low field ionization measurement e.g. CDMS I and II, EDELWEISS (next talk) - or photon (CRESST II)
More information on rare events But, operation at very low temperature!
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CDMS II Overview
Measure simultaneously ionization and athermal phonons
Most background sources (electrons, photons) scatter off electrons
WIMPS (and neutrons) scatter off nuclei Identify nuclear recoils event by event!
Bulk Electron Recoils ( 133 Ba) Bulk Electron Recoils ( 133 Ba) Nuclear Recoils ( 252 Cf) Nuclear Recoils ( 252 Cf)
•
Surface events:
Ionization Yield = E Q /E R Electrons produced by radioactive beta decays for electron recoils
Electrons ejected from nearby material Y~ 0.3 (Ge) for nuclear recoils
Gammas interacting within ~10
m
m of the surface
•
Events occuring near the surface (<~10
m
m) have an incomplete charge collection (“dead layer”) and can be misidentified as nuclear recoils
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I bias
The CDMS ZIP Ionization & Phonon Detectors
SQUID array R bias R fe edba ck Phonon D
250 g Ge or 100 g Si crystal 1 cm thick x 7.5 cm diameter Photolithographic patterning Collect athermal phonons
•
4 quadrants provide information on xy position
D C A B •
Informations on phonons pulse shape (ex. risetime), delay between charge and phonon pulses
z y x V qbia s
Q ou ter Q
inne r
inner
6 detectors stacked together
Allow detection of multiple interactions (neutrons) 1
m
tungsten 380
m
x 60
m
aluminum fins
2 runs done:
• •
Oct. 2003 - Jan 2004: 1 tower (4 Ge and 2 Si) Mar – Aug 2004: 2 towers (6 Ge and 6 Si)
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Z-Position Sensitivity Rejects Surface events
Gammas Surface event:
Energy deposited near the surface gives rise to slightly lower-frequency phonons
undergo less scattering and hence travel ballistically
Shorter risetime than bulk events Neutrons Bulk event Surface event Signal region
Overall rejection of surface events appears >99%
We are only beginning to take full advantage of the information from the athermal phonon sensors!
Improving modeling of phonon physics Extracting better discrimination parameters (timing and energy partition)
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CDMSII second run results
Second Run with 2 towers : 74.5 live days 96.8 (31.0) kg-days Ge (Si) before cuts 1 (0) event passed all cuts in Ge (Si) consistent with background estimate also this event occur during a time of known poor detector performance Si blind analysis Ge blind analysis Expected Background: (7-100 keV recoil energy
)
Surface events : 0.4
±0.2±0.2 for Ge; 1.2±0.6±0.2 for Si Neutron: 0.06 for Ge; 0.05 for Si Passed timing cuts
outside
nuclear recoil band Passed timing cuts
inside
nuclear recoil band (= WIMP candidate)
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CDMSII spin-independent limit
Upper limit on the WIMP-nucleon spin-independent cross-section is 1.6x10
-43 cm 2 for a WIMP with mass of 60 GeV/c2
.
Exclude large regions of SUSY parameter space MSSM example from.Bottino et al., 2004 CMSSM example from Ellis et al. (2005) DAMA 3
and 90% CL. Scattering on Na Gondolo & Gelmini (2005
) Phys. Rev. Lett. 96 (2006) 011302 23
CDMSII spin-dependent limit
CDMS has sensitivity to the spin dependent cross sections as well!
Limiting case of pure
neutron
coupling 29 Si and 73 Ge include an unpaired neutron (4.68%,) (7.73%)
neutron
Limiting case of pure
proton
coupling 73 Ge has nuclear excitations with non zero
proton Zeplin I SuperK indirect
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What’s next for CDMS?
CDMSII
•
5 Towers now installed
30 detectors: 19 Ge (4.75 kg) and 11 Si (1.1 kg)
•
currently commissioning will run through 2007
Toward 1 ton experiment: SuperCDMS
• •
3 phases: 25 kg - 150 kg – 1 ton of cold Ge det.
move from Soudan to Snowlab (reduce muon flux by 500)
•
increase detector thickness: 1 inch (0.64 kg) instead of 1 cm (0.25 kg)
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CRESST II: Phonons and Scintillation
Experiment located at Gran Sasso (3500 mwe)
Nuclear recoils have much smaller light yield than electron recoils
Photon and electron interactions can be distinguished from nuclear recoils (WIMPs, neutrons, ...)
Particle Mirro r CaWO Phonon Detector Thermometer 4 Light Detector Thermometer
No problem of surface events Very small scintillation signal Scintillation threshold will determine minimum recoil energy
Neutrons interact mainly with oxygen
WIMPs interact mainly with tungsten
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CRESST II: Phonons and Scintillation
Results from 20.5 kg-d exposure of two 300-g CaWO 4 prototypes (Jan 31 – Mar 23, 2004)
• •
No neutron shielding Observe 16 low-yield events consistent with neutron rates and oxygen cross section & light yield
•
Tungsten light yield not distinct from noise: claim background-free low-yield region 12-40 keV for 10-kg-d exposure of better detector (“Daisy”) Astropart. Phys. 23 (2005) 325 (astro-ph/0408006) Neutrons on oxygen
CRESST II upgrades March 2004 operation stopped to install - neutron moderator, muon veto - 66 readout channels (10 kg target) 90% W recoils Below this line Currently working on detector holder system and new analysis software, expect to be taking data by end summer 2006
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Liquid Xenon Detectors
• • • •
Potential to challenge cryogenic detectors
•
High atomic number (A=131) gives a high rate due to
WIMP-Nucleon
A 2 (if E is low) High density (~ 3g/cm 3 liquid) High light (175 nm) and ionization yield Easy to scale up to large volume Can be highly purified Time
Dual-phase LXe Time Projection Chamber (TPC) PMTs
Challenges of Liquid Xenon
•
Implementing “dual-phase” to improve scintillation signal near threshold
•
Ionization signal/noise poor near threshold
5 µs/cm ~1 µs width
Primary
~40 ns width
e -
LXe
E s
gas
E d B.A.Dolgoshein, V.N. Lebedenko, B.U. Rodionov, JETP Lett. 11 (1970) 513.
t
Several programs: XMASS, ZEPLIN, XENON, also experiments using Argon, Neon, He
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UK/UCLA Collaboration: Zeplin I
Located in UK Boulby mine (2800 mwe)
Three runs totaling 293 kg days exposure
Discrimination factor is pulse shapes
No in situ neutron calibration performed
0.1
1
Max sensitivity was 1.1x10
-42 cm 2 (Astropart 23 (2005) 44)
0.01
0.001
• • • •
5kg LXe target (3.1kg fid) 3 PMTs Cu construction Polycold cryogen cooling
Discrimination parameter
Gamma rays (Fast pulses)
0.0001
0.00001
1 1 0.1
0.01
0.001
0.0001
0.00001
• •
1 tonne Compton veto PMT background tag
1 10
pulse time constant (ns) +neutrons (Slow pulses)
10 100 100 29
ZEPLIN II Zeplin II, III, and beyond ZEPLIN III ZEPLIN IV/MAX
PMT Gas phase Liquid target
2 phases liquid and gas Xe
Calibrations and system checkouts underway in Boulby mine
Expect to run 2007 to 2012
Same 2 phases as ZEPLIN II but with 31, smaller 2’’PMTs at the bottom of the detector
Total mass 8 kg
Calibrations and system checkouts done at the surface
Expect to run 2007 to 2012
Start planning 1 ton experiment
To be installed in SNOlab 2008-2012
Explore capabilities of nobel gases with new ideas and design (2013 and beyong)
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XENON Experiment
Dual Phase Liquid/Gas Xe
The XENON design is modular Multiple 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass
1-ton scale experiment.
The 100 kg fiducial LXe volume of each module is shielded by additional 50 kg LXe. 100 kg LXe
A prototype (total mass 15 Kg LXe) is currently being shipped to Gran Sasso for assembly
Plan to begin running with shield in may 2006
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Bubble Chamber Revival
Example of CUOPP Experiment
Principle: Superheated liquid (e.g. CF b I)
•
Idea arose from superheated droplet experiments (SIMPLE/PICASSO)
•
Requires nucleation energy to overcome surface tension and form bubble
•
Only high-ionization energy density tracks from nuclear recoils sufficient to cause nucleation Insensitive to gammas, betas, & minimum ionizing particles CUOPP Setting up now at 300 mwe site at Fermilab Eventually will go to Soudan
•
Demonstrated bubble rates consistent with neutrons from cosmic rays at shallow site with 1 liter prototype
Challenges
•
No energy information for given event Must do multiple exposures at different pressures to derive energy spectrum
•
Possible alpha backgrounds; Operational stability
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Cathode
DRIFT: Look for diurnal modulation
Scattered WIMP Recoil Atom CS 2 Drift direction E-Field Recoil Electron
DRIFT I
•
Cubic meter in Boulby since 2001
•
Engineering runs completed
DRIFT II extension to 10 kg module proposed
Sensitive to direction of recoiling nucleus
•
Diurnal modulation signal – galactic origin of signal
Drift negative ions in TPC
• •
No magnet Reduced diffusion
Electron recoils rejected via dE/dx 40 keV S recoil in 40 Torr CS 2 (this is a simulation)
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Summary and Projections
Dark matter
• • •
Basic component of Universe Physics outside SM: Wimps, axions, etc.
Search: broad range of approach (indirect, direct detection)
•
Competitive reach for SUSY with LHC
Several direct-search technologies on line and leading to steady progress
• •
CDMS, Edelweiss, Zeplin Dama/Libra – systematics? Non-standard signal?
•
Many other experiments
Expansion to ton-scale
• • • •
Excellent prospects to see signal soon!
ZepMAX, SuperCDMS, XENON… EURECA (CRESSTII + Edelweiss) Unfortunately, costs are also growing
Field will likely contract to a few big experiments.
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