searching for dark matter with a bubble chamber michael b. crisler fermi national accelerator...
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Searching for Dark Matterwith a Bubble Chamber
Michael B. CrislerFermi National Accelerator Laboratory
14 February, 2009
Kavli Institue for Cosmological PhysicsUniversity of Chicago
Juan Collar† (PI), Luke Goetzke, Brian Odom, Nathan Riley,
Hannes Schimmelpfennig, Matthew Szydagis
Fermi National Accelerator LaboratoryStephen Brice, Peter Cooper, Michael Crisler, Lauren
Hsu, Martin Hu,Erik Ramberg, Andrew Sonnenschein, Robert Tschirhart
ChicagolandObservatory for
Underground Particle
Physics
Department of PhysicsIndiana University South Bend
Ed Behnke, Ilan Levine(PI), Tina Marie Shepherd
~85% of the Matter in our Universecan not be explained
Photo http://www.spacetelescope.orglumimous matter is only 3%
Fritz Zwicky
• Discovered anomaly in the motion of galaxies in clusters (in the 1930’s !)
• Suggested “dark matter” as the explanation
Rotation curve of our Solar System
From: http://www.astro.psu.edu/users/niel/psiwa/psiwa.html
Hydrogen Rotation Curve for the Milky Way
• Vera Rubin Galactic Rotation Curves• 1960’s and 1970’s• Strong Evidence for dark matter in galactic
halo
Matter Formation in the Big Bang
• Start with hot dense “soup” of elementary particles and radiation
• Expand, cool, “freeze out”
• Predictions for light element abundance
• Cosmic microwave background
• Strict upper bound on baryon content• Evidence for non-baryonic dark matter
Agreement on the Numbers:
• Gravitational Lensing provides additional graphic evidence for dark matter
• All techniques converge:• 3% luminous conventional matter• 14% dark conventional matter• 83% non-baryonic dark matter
Spectacular confirmation
http://chandra.harvard.edu/photo/2006/1e0657/
WIMP hypothesis
• Weakly Interacting Massive Particle
• WIMPs freeze out early as the universe expands and cools
• WIMP density at freeze-out is determined by the strength x of the WIMP interaction with normal matter
• Leads to x ~ weak interaction
What we DO NOT know…
• The WIMP mass Mx – prejudice 10<Mx<10000 Gev/c2
• The WIMP Interaction Cross-Section– Prejudice weak
– (give or take several orders or magnitude…)
• The nature of the interaction– Spin coupling?– Atomic Number coupling?
~107 WIMP’s per second
What we DO know• Halo density
particle flux x = 0v/mx
~105/cm2/sec for 100 GeV
0 = 0.3 GeV/cm3 a = 6.4 kpcr0 = 8 kpc
v0 = 220 km/sec
(r) 0
a2+r02
=a2+r2
f(v) d3v =3/2 v0
3exp(v2/v0
2)d3v
1
particle density 0= 0.3 GeV/cm3
• Velocity
(~3) 100 GeV WIMPs per quart
Collision velocity = 300 km/sec
WIMP kinematics:
8
‘
mWIMP vWIMP
v’WIMP
mN
vRecoil
vRecoil = vwimp cos 2 mWIMP
(mWIMP+mN
)
300 km/sec
=10-3
Erecoil~ ½ mN v2WIMP ~ ½ mNc
2 2
10’s of keV
10’s of GeV/c2
Natural Radioactivity
dark matter signal
4.679 MeV
81 KeV nuclear recoil
recoils from a-decays sit just beyondthe dark matter search region
most natural radioactivityis gamma and beta
It’s all about distinguishing electronRecoils from nuclear recoils…
Enter the Bubble Chamber…
Bubble Nucleation in Superheated Liquids by Radiation (Seitz, “Thermal Spike Model”, 1957)
Rc
Ec
RadiusW
ork
PP fluidvapor
CR
2
r
pfluid
pvaporSurface tension
Eloss
Sensitivity determined by vessel pressure, temperature
Nuclear Recoil Discrimination Through dE/dX
(psi)
plateau
protons
electrons
Waters, Petroff, and Koski, IEEE Trans. Nuc. Sci. 16(1) 398-401 (1969)
Plot of event rate vs. “superheat pressure” (= vapor pressure - operating pressure)
Superheating “just right” …one bubble per nuclear recoil
High degree of superheating =
electron sensitive, traditional bubble chamber
Superheating too low…no bubbles at all
note dE/dx ~ z2/2 where z = charge, =velocity/c
Nuclear Recoil discrimination better than 1/109 !
The Keys to Stability
• Use only the smoothest surfaces– quartz works well
• Clean the surfaces thoroughly
• Neutralize surface imperfections using a buffer liquid
• 12 cc Prototype Chamber
• 1 liter Prototype Chamber
1-liter ExperimentInstallation
in MINOS area
Photo Fermilab Visual Media Services
Triple Neutron Scatter
Muon Track @ 160 psi Superheat Pressure
Single Bubble Event
Event Reconstruction
Data from COUPP Mechanical Prototype
Solid lines:Expected WIMP response for
SD(p)=3 pb
Radon background
Energy Threshold In KeV
Single bubble events
Improved Spin-Dependent WIMP Limits from a Bubble Chamber, E. Behnke et al., Science 319:933-936,2008
Data from COUPP Mechanical Prototype
• competitive sensitivity for spin-dependent scattering, despite high radon background
Spin-dependent Spin-independent
Improved Spin-Dependent WIMP Limits from a Bubble Chamber, E. Behnke et al., Science 319:933-936,2008
New 30-liter chamber to operate in 2009
• 30 liter active volume
• Synthetic silica vessel
• High purity welding
• Gold wire seals
• Acoustic sensors to identify -particles via sound signature
• High-purity fluid distillation and handling system
• Deep underground site30 cm
120 cm
Bubble Chamber Summary
Spectacular /n discrimination >109 !
Excellent spatial resolution ~530 …eliminates spurious surface events
Compare multiple target nuclei (I, Br, F, Xe)
Excellent sensitivity to neutron multiple scattering events
Conclusion:
Bubble Chambers may be the next big thing
in Dark Matter Detection
Questions?