Dark matter
Detection methods
Experiments
Results
Ionisation (Charge)
Scintillation (Light) Phonons
(Heat)
Recoil
dark energy (73%)
CDM (23%)
Atoms (4%)
Does dark matter matter?
WMAP best fit
'Matter not visible to us because it emits no radiation that we can observe, but it is detectable gravitationally.'
What is dark matter?
says:
Dark matter Not dark matter
Particlephysicists say:
Most probable dark matter candidates are
W eakly I nteracting M assive P articles
in particular the lightest supersymmetric particle
the neutralino χ
A WIMP
χ
is ...produced in the Big Bang ...very long-lived or stable...like a massive neutrino (~ 100 mProton)
...very weakly interacting
1015 through a human body each day only < 10 interact
the rest is passing through unaffected
...highly abundant (~ 10 - 100 WIMPs per liter) ...moving at ~ 230 km/s w.r.t us
χ has exactly the right properties to be the dark matter
Detection methods:
Colliders
Indirect searches
Direct searches
Colliders
It is hoped/expected that the LHC (planned start-up in 2007) will be able to produce neutralinos,
however, it will not be able to constrain its cross section and identify it as the cosmologically observed dark matter.
Will deliver important information required to fully identify and
understand the particle.
A section of the LHC-accelerator in the testlab
Indirect searches
WIMPs are expected to be gravitationally captured in high density regions as galactic centers, the center of the Sun or the Earth.
Indirect dark matter searches assume that the WIMP is its own antiparticle (as predicted by SUSY models) or that equal
numbers of WIMPs and anti-WIMPs are present.
Therefore one should be able to detect a larger WIMP
annihilations signal from such regions, which can be manifested as a flux of gamma-rays, neutrinos or antimatter.
See other talks for information on the detection methods.
The Milky Way in gamma-rays
M ajor A tmospheric G amma I maging C herenkov
Direct searches
χ
WIMPs scatter elastically with target nuclei in the detector.
Expected counts per recoil energy:
with and
Detector physics F2 form factor
mn target nucleus mass NT number of target nuclei
ER recoil energy minor uncertainties
Astrophysics
n0 =ρ0 /mw local halo density
f(v) WIMP velocity distr.
vmax escape velocity
estimates
Particle physics mw WIMP mass
σ0 WIMP elastic scattering cross section
unknown
Ionisation (Charge)
≈ 20% detected energy
Scintillation (Light)
≈ few % detected energy
Phonons (Heat)
≈ 100% detected
energy Recoil
How to detect a nuclear recoil:
Difficulties with direct detection experiments:
The recoil energy is small (tens of keV)
Since the WIMP signal is expected to originate from elastic scattering, a featureless, quasi exponentially decreasing energy spectrum will result.
Detector needs to have an as low energy threshold as possible
The interaction rate is low
Need high target masses in order to gain a sufficient statistic in a reasonable life-time of the experiment.
Extremely high background
...
n
χ
β γ γ
γ γ γ
γ γ
γ
γ γ
γ γ
γ
γ γ β
β β
β β
β β
β β ββ
β
β β
β β β
β
β γ γ
γ γ
γ γ
γ
γ γ
γ γ γ
n n
n n n
Backgrounds for dark matter experiments:
electrons (β) photons (γ)
neutrons (n) nuclear recoil
}
electron recoilIonisation (Charge)
Scintillation (Light) Phonons
(Heat)
Combining two detection methods one can distinguish between nuclear and electron recoil
Can reduce the background to neutrons
Neutron sources:
Neutrons from detector components ultra-pure materials, veto
Neutrons from rock
passive shielding Neutrons from radon
Radon removal, gas-tight sealing and ventilation with radon-free air, careful treatment at the surface
Neutrons from cosmic-ray muons large depth, veto
Cosmic-ray muons
Image of the shadow of the Moon in muons as produced by the 700 m subterranean Soudan 2 detector in the Soudan mine in Minnesota.
Need an internal shielding close to the detector to protect it from the background (from rock, ...).
Because of its high atomic number and very low intrinsic radioactivity lead provides good shielding. Lead extracted from mines is however polluted by naturally radioactive
elements such as uranium and thorium which decay into Pb210. Can clean the lead from everything but Pb210.
Roman lead
has been extracted from the grounds several centuries ago.Pb210 has decayed (thalf ~ 22 yrs).
This lead shows extremely low activity.
Ingots of 'less archeological interest' of an ancient wreck that run agound off the Sept Iles (France).
Shielding
Experiments
Ionisation (Charge)
Scintillation (Light) Phonons
(Heat)
CRESST, Rosebud DAMA, ZEPLIN
CRESST, Rosebud
TPC, DRIFT
CDMS, EDELWEISS DAMA/LIBRA, ZEPLIN
C ryogenic D ark M atter S earch
at the Soudan lab in Minnesota
C ryogenic D ark M atter S earch
Absorbers:
250 g Ge or 100 g Si crystal 1 cm thick x 7.5 cm diameter
ZIP detector in its mount
Qinner Qouter A
B D
C Rbias Ibias
SQUID array Phonon D
Rfeedback
Vqbias
Z-sensitive Ionisation and Phonon mediated detector
WIMP hits a Ge/Si nucleus the nucleus recoils and vibrates the whole crystal.
This breaks Cooper pairs in the thin aluminum layer which build quasiparticles that diffuse through the Al-fins and are trapped in the tungsten TES where they release their binding energy to the W electrons T raises R increases
Phonon detection
ZIP with 4144 QETs
(quasiparticle-trap-assisted electrothermal-feedback transition-edge sensors)
380 μm x 55 μm Al fins (300 nm thick) 250 μm x 1 μm W
(35 nm thick)
passive tungsten grid
WIMP hits a Ge/Si nucleus breaks up e--hole pairs in the crystal; these get separated by E-field charge is collected by electrodes on the surface of the crystal
Events within a few μm of the surface experience a deficit in the charge collection (due to the detectors 'dead layer'). More on that soon...
Ionization signal
holes electrons
TES side
Vbias
readout
Discrimination capability I
gamma & neutron source (252Cf) only gamma source (60Co) no missidentification above the detector threshold (10.4 keV)
Discrimination capability II
Particles (electrons) that interact in the surface 'dead layer' of the detector result in reduced ioniza- tion yield and could be missiden- tified as nuclear recoils.
Use phonon rise time for discrimination of surface electrons
This eliminates almost all electron recoils while keeping most of the nuclear recoils.
Combined with ionization yield rejects
> 99.9999% of gammas
> 99% of 'betas'
neutrons gammas
surface electrons
ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si)
SQUID cards FET cards
4 K 0.6 K 0.06 K 0.02 K
Tower 1 Arrange 6 detectors in a tower check for multiple scattering (characteristic for background) Use both Ge and Si detectors
WIMPs cross-section ~ 5 times higher in Ge than Si but neutron cross-section similiar
CDMS II project
manager Dan Bauer of Fermilab holding one of the towers
Use several towers simultaniously
increase interaction probability/time
1st tower took data Oct 03 – Jan 04 March – Aug 04: 2 towers
Starting summer 2005:
tower 1-5
19 Ge (4.75 kg) & 11 Si (1.1 kg) Tower 6 & 7 expected to be
ready in 2008
State-of-the-art technology!
The cryostat uses six nested layers to cool the detectors to <50 mK.
This reduces the background vibrations of the detector's atoms and makes them more sensitive to individual particle collisions.
Cryogenics
"3Helium – 4Helium" dilution refrigerator
Energy is required to transport helium-3 atoms from the 3He-rich phase into the 3He-poor phase. Continuous crosssing of this boundary
Cooling of the mixture
outer polyethylene shield
Different layers of shielding around the icebox
muon veto paddles
inner lead shield
inner polyethylene shield
outer lead shield
icebox
ZIP
detectors
The future: SuperCDMS
Phase A: 25 kg (2008/ $50M) Phase B: 150 kg (2010/ $70M) Phase C: 1000 kg (2015/ $500M) (compare to < 6 kg today)
SuperCDMS is approved to be installed in SNOLab. A space is approved and ready for occupancy in 2007.
A factor of 1000 reduction in muon flux
= 1000 x less neutrons
particle DA rk MA tter searches
with highly radiopure scintillators at Gran Sasso
particle DA rk MA tter searches
with highly radiopure scintillators at Gran Sasso
9 scintillating thallium-doped sodium iodide (NaI) crystals of 9.7 kg each electrons and nuclei recoiling after a
collision cause emissions of photons that are detected using photomultiplier
tubes
electromagnetic background rejection by pulse shape discrimination (only)
1996-2002
60º 230 km/s 30 km/s
Milky Way Sun
Earth
Annual modulation
larger WIMP flux in June & smaller WIMP flux in December
Requirements of the annual modulation:
Modulated rate according cosine In a definite low energy range With a proper period (1 year)
With a proper phase (about June 2nd) For single hit in a mulit-detector set-up With modulated amplitude in the region
of maximal sensitivity < 7%
DAMA observed annual modulation over 7 years
P(A=0) = 7 x 10-4
Bernabei et al. astro-ph/0307403
From the DAMA homepage:
...
If you can bear to hear the truth you've spoken twisted by knaves to make a trap for fools,
...
...you'll be a Man my son!
(from "IF" by R. Kipling)
DAMA
results seem to be ruled out by all other experiments'results'
Upper limits starting to get into the interesting regions in mWIMP-σ-space In the near future one will be able to rule out certain SUSY-theories.
High demands on instrumentation!