Experimental dark matter searches

1
Experimental dark matter searches
2
Weakly Interacting Massive Particles
A WIMP c is like a massive neutrino produced
when T gtgt mc via annihilation through Z ( other
channels) annihilation/pair creation maintain
thermal equilibrium If interaction rates high
enough, the density drops as exp(- mc/T) as
T drops below mc annihilation continues,
production becomes suppressed

• But, weakly interacting
• ? may freeze out
• before total annihilation if
• gt Gann nc ??ann v??
• i.e., if annihilation too slow to keep up
• with Hubble expansion
• Leaves a relic abundance
• ?c h2 ??10-27 cm3 s-1 ????ann v??

freeze out
?
if mc and ?ann determined by electroweak physics,
then ?c?? 1
3
Detecting WIMPs
?0

• Direct detection
• WIMPs elastically scatter off nuclei
  nuclear recoils
• Measure recoil energy spectrum in target
• Indirect detection
• WIMPs annihilate
• in halo e, p, g
• in Sun, Earth core high energy ns

?
v/c ? 10-3
?0
4
Direct detection
WIMPs elastically scatter off nuclei in targets,
producing nuclear recoils, with ?nc 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)
WIMP flux
Amplitude of recoil energy spectrum, i.e. event
rate, normalized by ?nc, local WIMP number
density, and nucleus-dependent A2F2 (Q)
s-wavescattering
Elastic ScatteringForm Factors
At low Q, scattering is coherent and A2.
Coherence lost as Q increases parameterized by
form factor.
5
WIMP nucleus cross section

• In MSSM/CMSSM (neutralino)

in general s 10-5 between and 10-11
pb sensitivity of current experiments 10-6
pb testing some models, will test more models in
future as sensitivity improves Accelerator
constraints shrink SUSY bounds mainly
constrained upper bound g-2 can provide
constraint on lower bound if tentative
disagreement due to SUSY
1 event kg-1 d-1
current experiments
1 event 100 kg-1 yr-1
detectors low threshold low background large
masses good event discrimination
6
WIMP signatures
Annual modulation WIMP Isothermal Halo (assume
no co-rotation) v0 230 km/s
WIMP wind
Earth 30 km/s (15 km/s in galactic plane)
7
Annual Modulation
Not distinguish between WIMP signal and
background directly From the amplitude of the
modulation, we can calculate the underlying WIMP
interaction rate
WIMP Signal
2
Background
June
June
Dec
Dec
June
June
Dec
Dec
8
WIMP signatures
Diurnal modulation
v0 solar motion
WIMP
WIMP
WIMPs
42o
vo
a
Nuclear recoil
The mean recoil direction rotates over one
sidereal day
The distribution of the angle a between the
solar motion and recoil directions peaks at
a180o
9
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
Background neutrons
WIMPS 40 GeV
10
Direct detection techniques
CRESST ROSEBUD CUORICINO
CRESST II ROSEBUD
CDMS EDELWEISS
HDMS GENIUS IGEX MAJORANA DRIFT (TPC)
DAMA ZEPLIN I UKDM NaI LIBRA
XENON ZEPLIN II,III,IV
Large spread of technologies varies the
systematic errors, important if positive
signal! All techniques have equally aggressive
projections for future performance But different
methods for improving sensitivity
11
Where do we stand?
DAMA 3s
1 event/kg/day Most advanced experiments start
to test the predicted SUSY parameter space One
evidence for a positive WIMP signal Not
confirmed by other experiments
CDMS
EDELWEISS
ZEPLIN I
Predictions Ellis, Baltz Gondolo, Mandic all
12
The DAMA/LIBRA experiment
LIBRA
DAMA

• 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
• -gt positive signal (4 s)
• 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
13
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

14
The CDMS II experiment

• 1 tower of 4 Ge and 2 Si ZIPs
• operated at SUF 2001-2002 gt 120 livedays
• gt 99.98 rejection of bulk electron recoils
  5-100 keV
• gt 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

Ge
Si
Ge Ge Ge Si Ge Si
Muon anticoincident background
15
CDMS and DAMA
assumptions of standard halo, standard WIMP
interactions CDMS results incompatible with
DAMA model-independent annual-modulation data
(left) at gt 99.8 CL even if all low-energy
events were WIMPs
predicted WIMP modulation
predicted WIMP spectrum alone
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
neutron spectrum fit
16
The CDMS II experiment
first 2 towers at the Soudan mine
(2030mwe) m-flux reduced by 104, n-flux by
300 first dark April 03! goal 5 towers, 4 kg Ge,
1.5 kg Si 0.1 events/kg/keV/yr
EDELWEISS
CDMS 03
CDMS Soudan
entrance to the mine
17
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 _at_ Erec gt 20 keV,
• 30.5 kg ? day _at_ Erec gt 30 keV
• Incompatibility with DAMA candidate
• (99.8 C.L.) confirmed with three different
• detectors and extended exposure

G. Chardin 2003
18
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 to 120 detectors
(36 kg Ge)
19
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

Neutron source
Gamma source
10-20keV
Background 40 dru _at_ 100keV implies 85Kr lt 10-17
atoms/atom (standard Xe used)
Fiducial Volume cut
20
The ZEPLIN experiment
ZEPLIN II at RAL, UK
ZEPLIN I
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
21
The DRIFT experiment

• 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 diffusion
• during charge-drift
• operates at 40 torr , 140 g target mass
• discrimination through dE/dx measrmnt
• Future DRIFT-II
• scaled-up DRIFT-I with full 3D readout
• x50 sensitivity
• RD higher-resolution readout,
• higher-pressure operation

22
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!
1 ton is needed in order to detect 10 events per
year at s 10-46 cm2
Predictions Bottino, Ellis, Gondolo
23
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