Experimental dark matter searches

Weakly Interacting Massive Particles

But, weakly interacting may freeze outbefore total annihilation if

ann~ nann vi.e., if annihilation too slow to keep upwith Hubble expansion

Leaves a relic abundance:

h2 10-27 cm3 s-1 ann v

if m and ann determined by electroweak physics,

then ~ 1

A WIMP is like a massive neutrino: produced when T >> m via annihilation through Z (+ other channels); annihilation/pair creation maintain thermal

equilibrium

If interaction rates high enough, the density drops as exp(- m/T) as Tdrops below m: annihilation continues, production becomes suppressed

freeze out

Detecting WIMPs

Direct detection:

WIMPs elastically scatter off nuclei nuclear recoils

Measure recoil energy spectrum in target

Indirect detection:

WIMPs annihilate

in halo: e+, p, in Sun, Earth core: high energy ’s

v/c 10-3

0

0

Direct detection

WIMPs elastically scatter off nuclei in targets, producing nuclear

recoils, with n related to ann

(same diagrams - via Z, h, H, and squarks)

Energy spectrum of recoils is exponential with E ~ 50 keV,

dependent on WIMP and target nucleus masses: Boltzmann

distribution (isothermal halo) + s-wave scattering (NR)

dRdQ

n nQ

exp Q / Q A2 F2 Q

Q 2m

2mN

m mN 2 v2

mN

106 50 keV

H,h,Z

q q

q q

q~

E exp E kT

E 1

Amplitude of recoil energy spectrum, i.e. event rate, normalized by n, local WIMP number density, andnucleus-dependent A2F2 (Q)

At low Q, scattering is coherentand ~A2. Coherence lost as Qincreases; parameterized by form factor.

I/Xe

GeSi

WIMP flux

s-wavescattering

Elastic ScatteringForm Factors

WIMP nucleus cross section

In MSSM/CMSSM (neutralino):

in general: : 10-5 between and 10-11 pbsensitivity of current experiments: ~ 10-6 pbtesting some models, will test more modelsin future as sensitivity improves

Accelerator constraints shrink SUSY bounds:mainly constrained upper bound g-2 can provide constraint on lowerbound if tentative disagreement due to SUSY

1 event 100 kg-1 yr-1

1 event kg-1 d-1

detectors:low thresholdlow backgroundlarge massesgood event discrimination

current experiments

WIMP signatures

Earth 30 km/s (15 km/s in galactic plane)

log

dN

/dE

reco

il

Erecoil

June

Dec

~3% effect

Junev0

galactic centerSun 230 km/s Dec.

Annual modulation:

WIMP Isothermal Halo (assume no co-rotation) v0~ 230 km/s

Combining earth and solar system motion around galaxy :

T Q v0

4veerf

vmin vev0

erf

vmin vev0

where ve v0 1.05 0.07cos2 t tp

1 yr

tp June 2 1.3 days

WIMP wind

Annual Modulation

Not distinguish between WIMP signal and background directly

From the amplitude of the modulation, we can calculate the underlying WIMP interaction rate

95

97

99

101

103

105

-0.5 -0.1 0.3 0.7 1.1 1.50

25

50

75

100

125

-0.5 -0.1 0.3 0.7 1.1 1.5

±2%

Background

JuneJuneDec Dec

WIMP Signal

JuneJuneDec Dec

WIMP signatures Diurnal modulation:

42o

WIMPs

v0: solar motion

The mean recoil direction rotates over one sidereal day

vo

WIMP WIMP

Nuclear recoil

The distribution of the angle between the solar motion and recoil directions: peaks at =180o

WIMP signatures

Material dependence:

WIMPs: Ge has ~6x higher interaction rate per kg than Si

neutrons: Si has ~2x higher interaction rate per kg than Ge

WIMPS 40 GeV Background neutrons

Direct detection techniques

CRESSTROSEBUDCUORICINO

DAMAZEPLIN IUKDM NaILIBRA

CRESST IIROSEBUD

CDMSEDELWEISS

XENONZEPLIN II,III,IV

HDMSGENIUSIGEXMAJORANADRIFT (TPC)

ER

Phonons

Ionization Scintillation

Large spread of technologies:varies the systematic errors, important if positive signal!All techniques have equally aggressive projections for future performanceBut different methods for improving sensitivity

Where do we stand?

DAMA 3

ZEPLIN I

CDMS

EDELWEISS

~ 1 event/kg/day

Most advanced experimentsstart to test the predicted SUSY parameter space

One evidence for a positive WIMP signalNot confirmed by other experiments

Predictions: Ellis, Baltz & Gondolo, Mandic & all

The DAMA/LIBRA experiment

At LNGS (3800 mwe)

9 x 9.7 kg low activity NaI crystals,

each viewed by 2 PMs

2 methods of backgrd discr:

PS; annual modulation

-> positive signal (4 )

What next?

update to LIBRA (250 kg)

improved backround (~few)

improved light yield

Installation completed;

analyze additional 3 yr

of DAMA data (finished Jan 02)

Day 1 = Jan 1, 1995

LIBRA DAMA

The CDMS II experiment

At SUF (16 mwe) /Soudan (2030 mwe)

uses advanced athermal phonon (TES)

measuring charge and phonons

discrimination

position resolution

surface event rejection

gamma source

neutron source

neutrons

gammas

electrons

The CDMS II experiment

1 tower of 4 Ge and 2 Si ZIPs

operated at SUF 2001-2002; > 120 livedays

> 99.98 % rejection of bulk electron recoils: 5-100 keV

> 99 % rejection of surface events: 10-100 keV

n background x 2.3 lower due to inner poly (as expctd);

20 Ge recoil single scatters, 2 Si single scatters,

2 triple scatter, 1 nnn double scatter; consistent

with all single scatters caused by neutrons first results submitted to PRL, hep-ex/0306001

SQUID cards

FET cards

Ge

Ge

Ge

Si

Ge

Si

GeSi

Muon anticoincident background

CDMS and DAMA

assumptions of standard halo, standard WIMP interactions

CDMS results incompatible with DAMA model-independent annual-modulation data (left) at > 99.8% CL even if all low-energy events were WIMPs

Best simultaneous fit to CDMS and DAMA predicts too little annual modulation in DAMA, too many events in CDMS (even for no neutron background)

CDMS data

predicted WIMP modulation

predicted WIMP spectrum alone

neutron spectrum fit

The CDMS II experiment

first 2 towers at the Soudan mine (2030mwe)-flux reduced by 104, n-flux by ~ 300first dark April 03!goal: 5 towers, 4 kg Ge, 1.5 kg Si 0.1 events/kg/keV/yr

No SUSY gm-2Baltz&Gondolo,PRL 86 (2001) 5004

SUSY gm-2Baltz&Gondolo,PRL 86 (2001) 5004

CMSSMEllis et al. (2001)PRD 63, 065016

EDELWEISS

CDMS Soudan

CDMS 03

entrance to the mine

The EDELWEISS experiment

In Frejus UL (4800 mwe); 320 g Ge crystals

measure thermal phonons + charge

EDELWEISS I: 1 kg stage fall 2000, first semester 2002,October 2002 - March 2003

total exposure:

13.8 kg day @ Erec > 20 keV,

30.5 kg day @ Erec > 30 keV

Incompatibility with DAMA candidate

(99.8% C.L.) confirmed with three different

detectors and extended exposure

G. Chardin 2003

The EDELWEISS experiment

New run started: improved energy threshold

≈100% detection efficiency at 10 keV ER

September 2003:

end EDELWEISS-I run

install EDELWEISS-II

21 320 g Ge-NTD detectors

7 thin film (NbSi) 200 g Ge detectors

Achieve factor 100 improvement

in sensitivity

100 l dilution cryostat for up to120 detectors (36 kg Ge)

The ZEPLIN I experiment

Operating at the Boulby mine (~3000 mwe)

Single phase, scintillation in LiXe, PSD

3.7 kg liquid Xe (3.1 kg fid vol)

1 ton liquid scintillator veto

75 d livetime, 230 kg d of data

Fiducial Volumecut

0.00001

0.0001

0.001

0.01

0.1

1

1 10 100

pulse time constant ns

0.00001

0.0001

0.001

0.01

0.1

1

1 10 100

pulse time constant (ns)

Gamma source Neutron source

10-20keV

0.00001

0.0001

0.001

0.01

0.1

1 10 100

fitted time constant ns

data

gamma cal

GD fit Background:

40 dru @ 100keV implies85Kr < 10-17 atoms/atom

(standard Xe used)

The ZEPLIN experiment

ZEPLIN II at RAL, UK

ER Ionisation

Excitation

Xe+

+Xe

Xe2+

+e-

Xe**+ Xe

+Xe

Xe2*

Xe*

2Xe 2Xe

175nm 175nmTriplet

27ns

Singlet

3ns

Future ZEPLIN I:

more data, low Kr Xenon

ZEPLIN II/III:

Ionization + scintillation, 2 phase Xe;

30 kg, 6kg high field

II: tested at RAL, UK, PMs being produced

to be installed at Boulby in 2003

ZEPLIN I

The DRIFT experiment

Cathode Electric Field

Scattered WIMP

Recoil Atom

Drift direction

MWPC Readout

Plane

CS2 Recoil

Electron

CS2

In the Boulby mine (3000 mwe)Resolve ionization tracks in a gas

TPC filled with low-pressure EN gas (CS2)Endcap sense-planes: determination of range, orientation & energy (via ionization)e--capture by CS2 suppresses diffusionduring charge-driftoperates at ~40 torr , 140 g target massdiscrimination through dE/dx measrmnt

Future: DRIFT-II scaled-up DRIFT-I with full 3D readout & x50 sensitivity

R&D: higher-resolution readout, higher-pressure operationcathode-readout of positive ionsallowing event localization away from wire planes

Gamma Region

Overlap Region

Neutron Region

gamma region

neutron region

overlap

The ‘far’ future

1 event/kg d: EDELWEISS, CDMS, ZEPLIN

1 event/kg yr: CDMSII, CRESSTII, EDELWEISSII, ZEPLINII

1 event/100 kg yr: future projects!

Predictions: Bottino, Ellis, Gondolo 1 ton is needed in order to detect10 events per year at = 10-46 cm2

The ‘far’ future

Project Discrimin Type Mass Location

CryoArray Yes Ge/Si

phonon/ioniz

1 ton NUSEL

CRESST/

EDELWEISS

Yes Ge, CaWO4?

phonon/ion/scint

100 kg - 1t Gran Sasso?

Zeplin IV Yes LiXe ioniz/scint

2 phase

1 ton Boulby

XENON Yes LiXe ioniz/scint

2 phase

1 ton

(10 x 100 kg)

NUSEL

DRIFT3 Yes + direction TPC (CS2)

negative ion

100 kg Boulby

GENIUS No Ge ionization in LiN

100 kg -1 ton Gran Sasso

Majorana No Ge ionization 500 kg NUSEL