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?