1

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

2

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

33

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….