1 searching for dark matter in the universe: direct (indirect) methods for the detection of weakly...
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Searching For Dark Matter in the Universe:
Direct (indirect) methods for the detection of Weakly Interacting Massive Particles (WIMPs)
Nader MirabolfathiUniversity of California,
Berkeley
PIC -2004
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Evidence of Dark Matter: At Galactic scales…
halobulge
disksun
• Rotation curve of spiral galaxies imply the presence of dark matter
Expect v2 1/r
Velocity is measured using atomic lines from stars or the 21cm H line for the hydrogen clouds around the galaxy
Bergstrom, Rept.Prog.Phys. 63 (2000) 793E. Corbelli & P. Salucci astro-ph/9909252
M
m
If WIMPs are the halo, detect them on earth via scattering on nuclei in targets
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• Cosmic Microwave Background
• Clusters of Galaxies
• Supernovae SN1a
• Large-Scale structure formation
Many different approaches:
All agree that matter makes up
approx. 27 % of the Universe and…
2003
Ωmatter
Ω
Evidence of Dark Matter: At Cosmological scales…
... Big Bang Nucleosynthesis, CMB, and Structure Formation require thatapprox. 85% of the matter is Non Baryonic Cold Dark Matter
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Many CDM candidates:
• SUSY neutralinos• Axions• Gravitinos• Kaluza-Klein states• ...
Standard Model of Cosmology
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Candidate: Weakly Interacting Massive Particles
Production = Annihilation (T≥m)
Production suppressed (T<m)
Freeze out
1 10
100 1000m / T (time )
~exp(-m/T)
• WIMP : produced when T >> m via annihilation through Z (+other channels).
• If interaction rates high enough, comoving density drops as exp(- m/T) as T drops below m :
• Annihilation continues• Production suppressed.
Freeze out when annihilation too slow to keep up with Hubble expansion
Leaves a relic abundance:
h210-27 cm3 s-1ann vfr
For ~0.3:
• M ~ 10-1000 GeV• A ~ electroweak
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Direct Detection of WIMPs
If WIMPs are the halo, detect them 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:
Erecoil
Log(
rate
)
Energy spectrum of recoils ~ falling exponential with <E> ~ 15 keV
Rate (based on n and ) is of the order of a fraction of 1 event /kg/day
Isothermal and spherical Maxwell- Boltzmann velocity distribution V0=230 km/s <V>= 270 km/s, = 0.3 GeV / cm3
WIMP detector
Measure recoil energy
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Experimental Challenges
Low (keV) energy threshold
Large target mass
Suppression of backgrounds from radioactivity and cosmic rays (,,, neutrons)
• Deep sites• Passive/active shielding
Discrimination of residual background• Use WIMPS signatures
WIMPs: Extremely small scattering rate, small energy of the recoiling nucleus, and subtle signatures…
WIMPs Signatures:
• Nuclear recoils, not electron recoils
• Absence of multiple scattering
• Annual modulation
• Directionality
Requirements:
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WIMPS Detection Methods (strategies)
0, neutrons
Nuclear recoil
Electron recoil
1) Increase the mass of the absorber and keep the background as low as possible.
But how to distinguish WIMPs?i. Cosmological signature for the WIMPs assuming
standard halo model.ii. Statistically remove the background.
2) Discriminate WIMPs against dominant back ground (, , ). EVENT BY EVENT
How?
i. WIMPs are interacting with nucleons whereas , , interact with electrons.
ii. Increase the mass.
Sensitivity improves by 1/(MT)1/2
Sensitivity improves by 1/(MT)
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Current Direct Detection Experiments
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DAMA-NaI Experiment
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NaI
NaI
NaI
NaI
PM
T
PM
T
CopperLeadPolyethelene
DAMA - 100 kg NaI Experimental Apparatus
• Very elegant experimental setup - in place >1996
• Low Activity NaI scintillator9 9.7 kg NaI crystals, each viewed by 2 PMTs
• Located at Gran Sasso Underground Lab (3.8 kmwe) + Photon and Neutron shielding
• Two modes of Background discrimination– Pulse shape
– Annual modulation: ~2% modulation amplitude
POSITIVE SIGNAL
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Annual Modulation of Rate & Spectrum
galactic center
v0
Sun 230 km/s
Earth 30 km/s (15 km/s in galactic plane)
log
dN/d
Ere
coil
Erecoil
June
Dec
~5% effect
Combining earth and solar system motion around galaxy:
T Q( ) =π v0
4veerf
vmin +vev0
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟ −erf
vmin −vev0
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
where ve =v0 1.05+0.07cos2π t−tp( )
1 yr
⎛
⎝
⎜ ⎜
⎞
⎠
⎟ ⎟
⎡
⎣
⎢ ⎢
⎤
⎦
⎥ ⎥
tp =June 2 ± 1.3 days
June
Dec.
WIMP Isothermal Halo (assume no co-rotation) v0~ 230 km/s
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Annual Modulation
• Not distinguish between WIMP signal and Background directly
• From the amplitude of the modulation, we can calculate the underlying WIMP interaction rate
0
25
50
75
100
125
-0.5 -0.1 0.3 0.7 1.1 1.5
Background
JuneJuneDec Dec
WIMP Signal
95
97
99
101
103
105
-0.5 -0.1 0.3 0.7 1.1 1.5
±2%
JuneJuneDec Dec
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Modulation Amplitude
• There is clearly a modulation (4 - compared to null hypothesis)
mean over 2-6 keVee(22 – 66 keV recoil)
DAMA 2000 paper Figure 2
DAMA 15,000 kg-day
DAMA 215,000 kg-day
DAMA 3 + 438,000 kg-day
Best fit to Ann Moddata alone
Best FitDAMA NaI/1-4
• Best-fit WIMP model’s expected annual modulation does not appear to fit data; lowest point of 3 contour is much worse.
• Why? Additional constraint applied during max likelihood analysis: DC WIMP signal implied by AC signal must not exceed observed DC count rate best-fit cross-section is decreased
MinimumDAMA NaI/1-4 (3)
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DAMA → LIBRA
• 3 more annual cycles acquired– 58,000 + 49,800 = 107,800 kg-d– 7 cycles total
• Improved DAQ– Multiple scatters?
• LIBRA– Large sodium Iodide Bulk for
RAre processes– 250 kg with improved radiopurity– Taking data. Results have not
been announced.
• Further R&D toward 1-ton– NaI(Tl) radiopurification started
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ZEPLIN
Zoned Electroluminescence and Primary Light In Noble gasses
Location: Boulby Mine UK: UKDMC
ZonEd Proportional scintillation in LIquid Noble gasses
Or
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Why Xe?
• Available in large quantities.• High atomic number (A=131) gives a high rate due to WIMP-NucleonA2 (if E is low).• High density (~ 3g/cm3 liquid).• High light (175 nm) and ionization yield.• Can be highly purified.long light attenuation (m).long free electron life time (~5ms).• Easy to scale up to large volume.• No long lived radioisotopes.
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Principle Of Detection
Excitation • Production and Decay of excited Xe2
* states:1)Through singlet (3ns)2)Through triplets (27 nS)• dE/dx determines the proportion of different channels=>Nuclear more dense give more singlets or faster
Ionization •Ionized state Xe2
+, recombine with e- => Xe2*
=>Above relaxationdE/dx determines the recombination time channels=>Nuclear recoils (ps scale) electrons (40 nS)
Nuclear recoils are faster
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• Recombination allowed.• Only scintillation signal measured.• Discrimination is based on the pulse shape.• Discrimination is statistical.
ZEPLIN I
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ZEPLIN I Results
30 keV 122 keV&136 keV90 keV
Linear response 1.5 p.e/keV
(E)=1.24E1/2
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ZEPLINI Results (continued)
•Fiducial mass =3.2 KgMean event rate 2Hz.•Trigger three fold coincidences at 1pe.•2keV threshold.•Light yield 1.5-2.5pe/keV.•Statistics 293 kg.day in Three runs.
2003
2002
2003 (SUF)
2002
2004
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
No in situ neutron calibration
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LTDs, phonon sensors and beyond!
Who?• CDMS (Cyogenic Dark Matter Search)• EDELWEISS (Expérience pour DEtecter Les WIMPS En Sites Sousterrain)• CRESST (Cryogenic Rare Event Search with Superconducting Thermometers)
Low Temperature Detectors
LTD-1 1987LTD-9 2001
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LTDs, phonon sensors and beyond!
Why? Advantages?• After an interaction (event), all the excitations transform to heat. Good resolution• Phonon excitation~10-6eV compare to ~1or few eV for conventional semiconductor detectors. Low threshold
How to measure: Two methodes
T=E/C C(T/D)3
T could be big even with keV interaction.Using thermometers (Mott-Anderson or Superconducting thermometers) to measure T.
Low T Density of thermally excited phonons (noise) is very low.But we need to collect phonons before they reach the Equilibrium in the absorber. At low T Electron-phonon interaction is more effective than ph-ph interaction evaporated thermometers (electron bath).
Temperature: Equilibrium Lattice excitations (phonons)
Advantages:• Detecting the overall T No position dependence. • Best resolution obtained with this kind of detectors:~100 eV at 5 MeV ?
Weak points: • CMass Hard to increase the detector mass.• Unable to reconstruct the history of evts.
Advantages:• Could reconstruct the history of an event.• Thermometer collects constant fraction of phonons independent of the absorber Mass.
Weak points:• Dispersion or position dependence of E.• Homogeneity of thermometers.
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d3
d3>
d4
d4>
d2
d2> d1
d1
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Comparison between the two types of signals
80
60
40
20
0
1.00.80.60.40.20.0
80
60
40
20
0
1.00.80.60.40.20.0
300
200
100
0
0.300.250.200.150.100.050.00
300
200
100
0
0.300.250.200.150.100.050.00
300
200
100
0
0.300.250.200.150.100.050.00
T=E/Ctotal
T=E/Cfilm
T=E/Ctotal
To cold bath To cold bath
A) Temperature measurement B) Phonon measurement
Absorber
ThermometerThermometer
Absorber
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Heat is not enough!
Need another measurement to achieve event by event discrimination.
• The amount of charge created in a Semiconductor after an event depends on the type of interaction: Quenching factor (Q).• Quenching factor for an electron recoil event (Most of the radioactive background) is bigger than for nuclear recoil events (WIMPs).• By simultaneously measuring the charge and heat, one can discriminate - event by event WIMPs from the background.
Charge?
This defines the principle of detectors for CDMS and EDELWEISS experiments.
Scintillation?
CRESST: The same principle but scintillation instead of charge.
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Electron recoil
Nuclear recoil
Dead layer
What is different between CDMS and EDELWEISS
• Collection E field needs to be very low ~3Volts/cm.Dead layer (~50 m) > than traditional SM detectors (~1 m). limits discrimination!• Most of the bkgnd falls into DL region.very important to deal with.
Solutions
Avoid surface event by:
1) Carefully dealing with surface contamination.2) Introducing a blocking layer against the charge
back diffusion Introducing an amorphous Si layer below charge electrodes. Decrease DL to < 10 m
Identify near surface events:
1) Using phonon signal. Only possible if athermal phonons measured. (CDMS current, EDELWEISS R&D)
2) Using charge signal rise time. Needs a large bandwidth electronics. (EDELWEISS R&D )
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•Use of Ge NTD thermistors : FET readout•The guard electrode ~50% volume
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ZIP 1 (Si)ZIP 2 (Si)ZIP 3 (Ge)ZIP 4 (Si)ZIP 5 (Ge)ZIP 6 (Si)
SQUID cards
FET cards
4 K0.6 K0.06 K0.02 K
• CDMS Soudan first result with towerI• Tower I: 4 Ge (250 g) and 2 Si (100 g)• CDMS now running two towers 6 Ge and 6 Si• Si and Ge combination helps to better understand the neutron bkgnd.
• Edelweiss 2002 1 Ge (320g) detectors No Si • Edelweiss 2003 3 Ge (320g)
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Shielding
Layered shielding (reduce , , neutrons)~1 cm Cu walls of cold volume (cleanest material)Thin “mu-metal” magnetic shield (for SQUIDs)10 cm polyethylene (further neutron moderation)22.5 cm Pb, inner 5 cm is “ancient” (low in 210Pb)40 cm polyethylene (main neutron moderator)
Active Veto (reject events associated with cosmics)Hermetic, 2” thick plastic scintillator veto wrapped around shieldReject residual cosmic-ray induced eventsInformation stored as time history before detector triggersExpect > 99.99% efficiency for all , > 99% for interacting MC indicates > 40% efficiency for -induced showers from rock
30 cm parafin, 20cm Pb ,1 cm CuNo active vetoDilution fridge : 17mK base.
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CDMS 2004 Results (Calibration cuts)
• Neutron calibration after the run and Systematically check for gamma (e-recoil) calibration.• Phonon position dependence removed.• Nuclear and electron recoil bands defined (+/- 2) • Phonon timing cuts defined with calibration data.• Guard charge electrode defined.• Veto coincident events defined (window 50 s).
4 Ge (850g) 2 Si (170 g) * 52 live days during 92 calendar days
• Selection criteria and nuclear recoil efficiency• Veto coincidence (50 us window) - 97%• Baseline stable (pileup, noise,…) - 95%• Nuclear Recoil band (2 sigma) - 95%• Phonon Timing cuts - 80%• Charge outer electrode cut - 75%• TOTAL - 53%
Electron recoil
Nuclear recoil
Charge spectrum
Phonon Spectrum
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WIMP search data with Ge detectors
Recoil energy (keV)
Char
ge y
ield
• Exposure– 92 days (October 11,
2003 to January 11, 2004)
– 52.6 live days
– 20 kg-d net (after cuts)
• Data: Yield vs Energy– Timing cut off
– Timing cut on
– Yellow points from neutron calibration
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WIMP search data with Ge detectors
Recoil energy (keV)
Char
ge y
ield
• Exposure– 92 days (October 11,
2003 to January 11, 2004)
– 52.6 live days
– 20 kg-d net (after cuts)
• Data: Yield vs Energy– Timing cut off
– Timing cut on
– Yellow points from neutron calibration
34
WIMP search data with Ge detectors
Recoil energy (keV)
Char
ge y
ield
• Exposure– 92 days (October 11,
2003 to January 11, 2004)
– 52.6 live days
– 20 kg-d net (after cuts)
• Data: Yield vs Energy– Timing cut off
– Timing cut on
– Yellow points from neutron calibration
No nuclear-recoil candidates
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Comparing Cross section-WIMP Mass plots
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Future
• The presented results are from one tower• CDMS II is now running two towers (6 *Ge (250 g) 6 *Si (100 g)• Background of the second tower is very similar to tower I.• Run stops mid July of this year • New three towers of detectors will be installed October this year• CDMS II ends by the end of 2005.
• March 2004 end EDELWEISS I• Install EDEWEISS II with 21*320 g Ge NTD+Install 7*400 g NbxSi1-x athermal phonon detectors (Dead layer rejection)• The 100 liter dilution fridge has been successfully tested. Capacity for 120 detectors or 35 Kg Ge
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CRESST : Scintillation/Heat instead of Charge/HeatGran Sasso
• Background discrimination by simultaneously measuring light/heat.• Uses a cryogenic detector (the same as phonon detector) for light measurement.• Works with different absorber materials: CaWO4 (mainly), PbWO4, BaF,..Advantage to change the absorber• Phonon channel:320 g CaWO4 (=40mm,h=40mm) , W-SPT (4*6 mm2).• Light channel:30*30*.4 mm3 W-SPT.• Reflector: Polymer foil, Teflon.
Need 33 Modules to complete CRESST II goal
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CRESST Sensitivity and rejection
• High rejection:99.7% E > 15 keV99.9% E>20 keV
• 9.7 kg.day data•Only half of the data analyzed.• Data without neutron shield.•Sensitivity limited by n.
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Future direct detection experiments
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DRIFT experiment
Directional Recoil Identification From Tracks• Standard halo model for WIMPs in our galaxy suggests that the axis of recoils changes in the 24 hours (earth). • Axis of recoil is a cosmological signature for WIMPs.• Ionization track in a low pressure gas (CS2) depends on the type of interaction (Discrimination).• Multi wire proportional chamber ?
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e-
C+,S+
WIMPS
E
SiTime of flightz
Principle of DRIFT
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• Low Prsure CS2 (40 Torr) 1 m3, 0.167 kg, 20 micron diameter wires 2 mm pitch.• 1 mm track for nuclear recoils• Many calibration runs with 55Fe (5.9 keV X-rays)• Neutron Calibration with 252Cf.• Polypropylene shielding (~ 50 cm).• Dark matter run started.• Energy threshold 15 keV.
gammas
C recoils
S recoils
DRIFT setup
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Discrimination in ZeplinII and III,IV,…Double phase Xe : Ionization
Calibration of the prototype with gamma and neutron sources showed very good
gamma/neutron discrimination (Cline et al. Astroparticle Phys. 12(2000) 373)
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ZEPLIN projected~3
Kg ZEPLIN I
~30k
g ZEPLIN II
~100
0 k
g
ZEPLIN IV
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Xenon: Perspective
• Dual phase Xe experimnent• Light/Ionization• Very-low BG PMT• Prototype 1 cm drift• 10 kg prototype underway• 100 kg phase : 1 TPC• Modular: each module 100 kg• Self protected by outer Xe
• 1 Ton scale• 99.5 % discrimination eff• 16 keV threshold
Reach
: ~10-
46 cm2
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WIMPS indirect detection experiments• AMANDA , ICECUBE (Southe po;e)• ANTARES• NESTOR• Superkamiokande, Hyperkamiokande• -ray telescopes: CANGAROO, MAGIC, HESS• Satellite experiments: AMS-02, GLAST
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WIMP indirect detection
•WIMP elastic scattering. But in average it will lose energy:
V<Vescape accumulates in the center of large massive objects like the sun earth or galaxy.
•Neutralino : Majorana particle its own anti particle.
•If massive annihilates.
•Annihilation ;b,c,t quarks;gauge and Higgs Bosons
,,,e+, p-.
•Signature:
search for excess of up-going muons
•Form direction from center of sun galaxy or Earth.
•Search for annihilation lines (galactic center, cosmological…)
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Neutrinos from the center of the earth, sun, galaxy.
Assumptions:•Dark matter in the galaxy due to
• Density~ 0.3 GeV/cm3
AMANDA, Super K…
49Amundsen-Scott South Pole station
South PoleDome
Summer camp
AMANDA
road to work
1500 m
2000 m
[not to scale]
AMANDA
50PMT noise: ~1 kHzOptical Module
AMANDA-II19 strings677 OMs
Trigger rate: 80 HzData years: 2000-
AMANDA
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Sensitivity to muon flux from neutralino annihilations in the center of the Earth:
WIMP annihilations in the center of Earth
Eμ > 1 GeV
Muon flux limits
PRELIMINARY→→ + H Z, W,,ll ,qq -xx
Look for vertically upgoing tracks
NN optimized (on 20% data) to - remove misreconstructed atm. μ - suppress atmospheric ν - maximize sensitivity to WIMP signal
Combine 3 years: 1997-99
Total livetime (80%): 422 days
No WIMP signal found
Disfavored by direct search(CDMS II)
→→ + -WWxx→→ + -ôôxx
Limit for “hardest” channel:
GeV 5000-100 =xM
GeV 50 =xM
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CDMS 2004
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Summary
• Direct detection Experiments (CDMS,EDELWEISS, CRESST..) have already explored the regions of the most optimistic SUSY models.
• Despite the lower amount of exposure (~20 kg.day compare to 110,000 kg.day), the event-by-event discrimination methods are giving the best sensitivities.
• Extremely high discrimination + large mass seems to be the only solution for the next generation of direct detection experiments.
• The current and next experiments (CDMSII, EDELWEISSII, ZEPLIN IV, XENON …) will explore the core of many SUSY models in few years.
• Indirect detection will be complementary but hardly competitive to low scalar WIMPs detection.
• The accelerator (LHC) results + the direct detection experiments will soon (not in the cosmological sense!) let to discover the nature of the dark matter.
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Modulation Animation in NaI
50 GeV WIMP
000904.4 rjg
Background
Sun movin
g throug
h WIMP Halo
Threshold
57Depth (mwe)
Log
10(M
uon
Flu
x)
(m-2s
-1)
Log
10(M
uon
Flu
x)
(m-2s
-1)
Depth (mwe)
Muon flux: 4/m2/dayNeutron flux:1.5e-6/cm2/s
Muon flux: 70/m2/day Neutron:
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WIMP search data with Ge detectors
Recoil energy (keV)
Char
ge y
ield
• Exposure– 92 days (October 11,
2003 to January 11, 2004)
– 52.6 live days
– 20 kg-d net (after cuts)
• Data: Yield vs Energy– Timing cut off
– Timing cut on
– Yellow points from neutron calibration
Well, maybe 1….