1. Searching for Dark Matter with a Bubble Chamber Michael B. Crisler Fermi National Accelerator Laboratory 14 February, 2009

2. Kavli Institue for Cosmological Physics University of Chicago Juan Collar † (PI), Luke Goetzke, Brian Odom, Nathan Riley, Hannes Schimmelpfennig, Matthew Szydagis Fermi National Accelerator Laboratory Stephen Brice, Peter Cooper, Michael Crisler, Lauren Hsu, Martin Hu, Erik Ramberg, Andrew Sonnenschein, Robert Tschirhart Chicagoland Observatory for Underground Particle Physics Department of Physics Indiana University South Bend Ed Behnke, Ilan Levine(PI), Tina Marie Shepherd

3. ~85% of the Matter in our Universe can not be explained Photo http://www.spacetelescope.org lumimous matter is only 3%

4. Fritz Zwicky Discovered anomaly in the motion of galaxies in clusters (in the 1930’s !) Suggested “dark matter” as the explanation

5. 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

6. 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

7. 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

8. Spectacular confirmation http://chandra.harvard.edu/photo/2006/1e0657/

9. 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

10. What we DO NOT know… The WIMP mass Mx –prejudice 10<Mx<10000 Gev/c 2 The WIMP Interaction Cross-Section –Prejudice  weak –(give or take several orders or magnitude…) The nature of the interaction –Spin coupling? –Atomic Number coupling?

11. ~10 7 WIMP’s per second What we DO know Halo density particle flux  x =  0 v/m x ~10 5 /cm 2 /sec for 100 GeV  0 = 0.3 GeV/cm 3 a = 6.4 kpc r 0 = 8 kpcv 0 = 220 km/sec  (r) 00 a 2 +r 0 2 = a 2 +r 2 f(v) d 3 v =  3/2 v 0 3 exp(v 2 /v 0 2 )d 3 v 1 particle density  0 = 0.3 GeV/cm 3 Velocity (~3) 100 GeV WIMPs per quart Collision velocity = 300 km/sec

12. WIMP kinematics: 8 ‘‘  m WIMP v WIMP v’ WIMP mNmN v Recoil v Recoil = v wimp cos  2 m WIMP (m WIMP +m N ) 300 km/sec  =10 -3 E recoil ~ ½ m N v 2 WIMP ~ ½ m N c 2  2 10’s of keV 10’s of GeV/c 2

13. Natural Radioactivity dark matter signal 4.679 MeV  81 KeV  nuclear recoil recoils from a-decays sit just beyond the dark matter search region most natural radioactivity is gamma and beta

14. It’s all about distinguishing electron Recoils from nuclear recoils…

15. Enter the Bubble Chamber…

16. Bubble Nucleation in Superheated Liquids by Radiation (Seitz, “Thermal Spike Model”, 1957) RcRc EcEc Radius Work r p fluid p vapor Surface tension  E loss Sensitivity determined by vessel pressure, temperature

17. 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 ~ z 2 /  2 where z = charge,  =velocity/c

18. Nuclear Recoil discrimination better than 1/10 9 !

19. The Keys to Stability Use only the smoothest surfaces –quartz works well Clean the surfaces thoroughly Neutralize surface imperfections using a buffer liquid

20. 12 cc Prototype Chamber 1 liter Prototype Chamber

21. 1-liter Experiment Installation in MINOS area Photo Fermilab Visual Media Services

22. Triple Neutron Scatter

23. Muon Track @ 160 psi Superheat Pressure

24. Single Bubble Event

25. Event Reconstruction

26. 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 319:933-936,2008E. Behnke et al.

27. Data from COUPP Mechanical Prototype competitive sensitivity for spin-dependent scattering, despite high radon background Spin-dependentSpin-independent Improved Spin-Dependent WIMP Limits from a Bubble Chamber, E. Behnke et al., Science 319:933-936,2008E. Behnke et al.

28. 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 site 30 cm 120 cm

29. Bubble Chamber Summary Spectacular  /n discrimination >10 9 ! Excellent spatial resolution ~530  …eliminates spurious surface events Compare multiple target nuclei (I, Br, F, Xe) Excellent sensitivity to neutron multiple scattering events

30. Conclusion: Bubble Chambers may be the next big thing in Dark Matter Detection

31. Questions?

32. Galactic Big Picture  (r) 00 a 2 +r 0 2 = a 2 +r 2 f(v) d 3 v =  3/2 v 0 3 exp(v 2 /v 0 2 )d 3 v 1  0 = 0.3 GeV/cm 3 a = 6.4 kpc r 0 = 8 kpc v 0 = 220 km/sec