1. Can We Detect Dark Matter With Bubble Chambers? Andrew Sonnenschein Fermilab 15 ft. Bubble Chamber

2. Cartoon of a Typical Galaxy Invisible material of unknown composition Stars and bright gas 20th century astrophysicists developed a bizarre model for galaxy mass distribution: >85% of total mass “Halo” ~ 200 kpc

3. Dark Matter in 20th Century: a Synopsis 1933 1940- 1970 1970’s 1980s 1990s 2003 Zwicky estimates Coma cluster mass using virial theorem and finds 500 times more than expected (but would have been only 40 times if he had used right value of Hubble constant) Problem of “missing mass” largely ignored for 30 years… Observation of galaxy rotation curves beyond optical radius show that rotation curve is still flat. Realization that the universe would not look right on large scales without dark matter. New mass estimators developed: binary galaxies, X-ray cluster maps, large scale motions… Could  m =1? (as required by inflation with no cosmological constant) (definition:  m =  matter /  critical ) “Golden Age of Cosmology” - vast quantities of data. New mass estimators: strong and weak gravitational lensing. COBE discoveres fluctuations in the cosmic microwave background (CMB). Revolutionary discovery that cosmological constant not zero by observing supernovae. First acoustic peak in CMB detected, confirms that universe is flat, so   +  m =1.  CMB + Supernovae:   = 0.7 and  m =0.3.  Measurements of deuterium & CMB strongly constrain baryon density:  baryons = 0.04. WMAP confirms consensus cosmological parameters. Need for non-baryonic dark matter firmly established.

4. What is the Non-Baryonic Part of the Dark Matter? NEUTRINOS At least we know they really exist. We think we know how many there are, ~ 300/cm 3 They have mass, but seem to be too light (<0.1 eV) to be the dominant DM component. Probably are about ~0.1% of the dark matter. Neutrinos in the few eV mass range would make wrong kind of large scale structure. WIMPS (Weakly Interacting Massive Particle) = best bet? Particles with neutrino-strength (or less) interactions but much heavier. Would be thermally produced in big bang and “freeze out” in an abundance determined by mass and self-annihilation cross section. Minimal supersymmetric standard model (MSSM) naturally provides a WIMP candidate, the neutralino: Expected to be stable. Linear combination of Bino, Wino, and two Higgsinos MSSM has many free parameters which determine exactly what its properties would be. + Axions, Wimpzillas, Primordial Black Holes, Cosmic Strings, Gravitinos…

5. Dark Matter in the Minimal Supersymmetric Standard Model Standard Model MSSM Higgs Superpartner flavor states Superpartner mass states u,d,s,c,t,b squarks e  sleptons e, ,  sneutrinos W±W± charged winos ±2±2 H±H± charged higgsinos ±1±1  photino 0404 Z0Z0 zino 0303 hh higgsino 0202 H 0101 A0A0 G gravitino Charginos Neutralinos mixing Lightest neutralino is dark matter? Mass: 30-1000 GeV

6. Based on simple assumptions: Particles are gravitationally bound to Halo, with Maxwellian velocity distribution (V rms =220 Km/s) and local density 0.3 GeV/cm 3 WIMPs are heavy particles, 10 GeV< M WIMP < 1 TeV. Nuclear scattering can efficiently transfer energy to a nucleus, since M nucleus ~M wimp. The signal will be a nuclear recoil, with energy ~10 keV Scattering is non-relativistic. Shape of spectrum does not depend on particle physics inputs. Amplitude of spectrum depends on unknown supersymmetry parameters and some astrophysical uncertainties. Spectrum of Wimps in a Detector on Earth Germanium detector

7. Non- Relativistic Scattering of Wimps on Nuclei For v/c~0.001, the “vector”, “axial vector”, “scalar” couplings become simply spin- dependent and spin-independent. Interaction is coherent over nucleus, because de Broglie wavelength of Wimp is big: For spin-independent coupling, cross section is proportional to A 2 (A = number of protons and neutrons). Spin-dependent cross section is suppressed by cancellation of pairs of opposite spin nuclei, so coupling is effectively to net total nuclear spin. So, it’s probably easier to discover Wimps via their spin-independent couplings, using target materials with heavy nuclei and taking advantage of A 2 coherence factor.

8. Event Rate [events/keV-Kg-day] Energy [keV] WIMP Mass [GeV/c 2 ] WIMP-nucleon Cross Section [cm 2 ] Excluded Region Largest compatible Signal for 1 TeV Sensitivity is Limited by Background Radiation Assume halo density 0.3 GeV/cm 3 Construction of exclusion plot is illustrated below with data from an early experiment.

9. Backgrounds from Radioactivity and Cosmic Rays ( figure from Brodzinski et al, Journal of Radioanalytical and Nuclear Chemistry, 193 (1) 1995 pp. 61-70) Gammas & betas From primordial, cosmogenic, and manmade nuclei: (not an exhaustive list!) 238 U, 232 Th + daughters (incl. 222 Rn) 40 K, 14 C 85 Kr, 137 Cs, 3 H - nuclear tests 68 Ge, 60 Co - cosmogenic in detector setups Cosmic Rays (p, , , e…) Can be reduced by going underground. The  ’s penetrate to great depth. Neutrons From  spallation or ( , n) reactions in rocks, with alphas from U/Th chains. Can be shielded with moderator at low energies. A long history of successful attempts to reduce by choosing special materials and shielding.

11. 40 keV Ar in 40 Torr Ar 13 keV e - in 40 Torr Ar WIMPs interact with the nucleus, while most backgrounds are due to electron scattering by gamma and beta rays. The resulting spatial distributions of energy and charge are very different. (Figures from DRIFT collaboration) Discriminating Against Backgrounds X [mm] Y [mm]

12. Observables Which Could be Used to Separate WIMP-nucleus Events from Backgrounds Spatial distribution of charge deposited in a low-pressure drift chamber. Ratio of ionization to deposited energy in a cryogenic detector (CDMS). Pulse shape discrimination in scintillating materials (organic & inorganic). Ratio of ionization to scintillation in liquid noble gases. Ratio of scintillation to deposited energy in a cryogenic detector. Annual modulation due to motion of Earth around the Sun. Daily modulation in direction of ion tracks in a low-pressure drift chamber, due to rotation of the Earth. Efficiency for bubble formation in superheated liquids.

13. Recoil Energy [keV] Charge Energy [keV ] Co-60  rays. Cf-252 neutrons. Background Discrimination With Simultaneous Charge and Energy Measurement in Cryogenic Detectors +  Rays, neutrons, & WIMPs Semiconductor - + - - - - - + + + phonons electrons & holes Temperature sensor E Charge collector CDMS-I

14. CDMS Ge and Si ‘ZIP’ detectors Superconducting tungsten transition-edge sensors Simultaneously measure Ionization and athermal Phonon energy signals Pulse timing/parameters give sensitivity to event location – Z position – reduction of surface backgrounds The detectors are operated in dilution refrigerators at ~20mK tungsten aluminum fins

16. Current CDMS-II Results (Sept. 2005) 6 Ge & 6 Si detectors x 75 days 10-100 keV energy window 1 candidate event with 0.4 expected in Ge No candidate events in Si Nuclear recoil region Passed all cuts Passed pulse timing/ shape cuts

17. CDMS Current Limits (Sept. 2005) For spin- independent channel

18. Lots of Parameter Space Left to Explore Assumptions in this plot include: Spin-independent coupling. Maxwellian halo velocity distrib. V rms = 220 km/s  dm =0.3 GeV/cm 3 MSSM Parameter Space from Gondalo et al., 1999. CDMS, EDELWEISS, ZEPLIN Limits (Feb. 2005) MSSM

19. A Selection of Past, Present, and Near-Future Experiments. Main Topic Most Sensitive as of Feb. 2006

20. Why Bubble Chambers for Dark Matter? 1. Large target masses would be possible. Multi ton chambers were built in the 50’s- 80’s. 2. An exciting menu of available target nuclei. No liquid that has been tested seriously has failed to work as a bubble chamber liquid (Glaser, 1960). Most common: Hydrogen, Propane But also “Heavy Liquids”: Xe, Ne, CF 3 Br, CH 3 I, and CCl 2 F 2. Good targets for both spin- dependent and spin-independent scattering. Possible to “swap” liquids to check suspicious signals. 3. Low energy thresholds are easily obtained for nuclear recoils. < 10 keV easy to achieve according to standard nucleation theory. 4. Backgrounds due to environmental gamma and beta activity can be suppressed by running at low pressure. This effect used for superheated droplet detectors (SIMPLE, PICASSO).

21. Why Do Liquids Not Always Boil When They Pass into the “Vapor” Part of the Phase Diagram? In the “vapor” region, the equilibrium state is a vapor. But liquids have surface tension, so there is an energy cost to create a bubble. This energy barrier may be greater than kt. a metastable (“superheated”) liquid state may continue to exist for some time. The liquid will boil violently once the energy barrier to the vapor phase is overcome.

22. Bubble Nucleation in Superheated Liquids by Radiation (Seitz, “Thermal Spike Model”, 1957) RcRc P vapor P external particle Surface tension  Pressure inside bubble is equilibrium vapor pressure. At critical radius R c surface tension balances pressure. Bubbles bigger than the critical radius R c will grow, while smaller bubbles will shrink to zero. Boiling occurs when energy loss of throughgoing particle is enough to produce a bubble with radius > R c RcRc EcEc Radius Work

23. dE/dX Discrimination in a Small Propane Chamber  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) (psi)

24. Bubble Chambers As WIMP Detectors The classical bubble chambers operated in the 50’s-80’s would not have been useful for dark matter detection, because they were only sensitive for a few msec at a time. Example: Operating pressure cycle for SLAC 1-meter hydrogen chamber. Ballam and Watt, 1977. ~ 10 msec recompression decompression

25. Bubble Nucleation in Cracks Liquid 0.1  m Solid nucleation sites Trapped gas volumes in surface imperfections are now known to be the primary source of nucleation. Most (all?) construction materials have rough surfaces at scales below 1  m, but some materials much better than others. Historically, problem was overcome for high energy physics experiments by rapid cycling of chamber in sync with a pulsed beam. Bubbling at walls was tolerated because of finite speed of bubble growth. A few small “clean chambers” (~10 ml) were built in the 50’s and 60’s, with sensitive times ~1 minute. Ways to preserve superheated state: Elimination of porous surfaces in contact with superheated liquid. Precision cleaning to eliminate particulates. Vacuum degassing. … a few other tricks borrowed from chemical engineers

27. Prototype High-Stability Bubble Chamber 3-way valve Propylene glycol buffer liquid prevents evaporation of superheated liquid. Acoustic sensor Quartz pressure vessel Glass dewar with heat-exchange fluid Piston Camera (1 of 2)

28. Possible Target Liquids Mass FractionsDensity Boiling Point @ 1 atm Comments CF 3 Br 8% C (Z=6) 38% F (Z=9) 54% Br (Z=35) 1.5 g/cc -58  C Good for spin-dependent and spin- independent couplings. CF 3 I 6% C 29% F 65% I (Z=53) 2.1 g/cc -23  C Spin-dependent and spin-independent Non- ozone depleting C3F8C3F8 19% C 81% F1.4 g/cc -37  C Spin- dependent only. Xe100% Xe (Z=54)3.0 g/cc -108  C Possible use in hybrid scintillating bubble chamber.

30. High Speed Bubble Chamber Movie 1000 frames/ second 241 Am-Be neutron source

31. Bad surface Good surface

32. 7 keV at -10  C

33. Calibration with Neutron and Gamma Sources Isotope sources: 241 AmBe, 88 YBe (neutrons) and 88 Y (gammas). Calibration based on maximum kinematically allowed nuclear recoil energy. Future: monoenergetic accelerator neutrons needed to cleanly measure efficiency. 2 mCi 88 Y source

34. Position Reconstruction Bubble positions can be reconstructed in 3 dimensions by scanning images taken by two cameras offset by 90 degrees. Position resolution is currently 530 microns r.m.s. (approximately 1/4 bubble diameter) Uniform spatial distribution of background events, consistent with cosmic ray neutrons. 163 background events (1.5 live days)

35. Background Rate 1 event/ (15 minutes) for 18 grams of CF 3 Br in sub-basement lab (~6 ft. depth). Observed rates are consistent with measured ambient neutron flux.

36. Neutron Multiple Scattering Multiple bubbles are present in approximately 4% of events in our “background” data set. These events can only be caused by multiple neutron scattering, since uniform size of bubbles implies simultaneous nucleation at multiple sites. Events such as this can be used to measure neutron backgrounds in- situ while searching for recoils due to WIMPs.

37. Background Due to Nuclear Recoils from Alpha Decay Alpha decay produces monoenergetic, low energy nuclear recoils. For example, consider 210 Po-> 206 Pb: 206 Pb  E  = 5.407 MeV E R = 101 keV The recoiling nucleus will nucleate a bubble in any chamber that is sensitive to the lower energy (~10 keV) recoils expected from WIMP scattering. The 238 U and 232 Th decay series include many alpha emitters, including radon ( 222 Rn) and its daughters. Radon is highly soluble in bubble chamber liquids.

38. Table of Isotopes Iodine Rare earth group Uranium and Thorium Decay Chains Fluorine 222 Rn protons neutrons betas Bromine

39. Counting Test Facility: A Prototype for Borexino Solar Experiment

40. Backgrounds for a Large Bubble Chamber Experiment AssumptionsTotal Event Rate neutronsDeep site with shielding<0.01/ton-day Gammas & betas“Rejection factor” better than 10 -9 <0.1/ton-day AlphasBest Borexino CTF level dominated by radon decay ~1/ton-day? Fluid handling techniques to be adapted from Borexino solar neutrino observatory. Background likely to be dominated by radon and its daughters. These levels are ~3 orders of magnitude lower than seen in current generation dark matter experiments.

41. test site ~300 m.w.e. at Fermilab

42. Neutron Flux Underground Heusser, 1995. NuMi Tunnel U. Chicago Sub-basement lab Dominant neutron sources: Direct cosmic ray neutrons. Neutrons from radioactivity in rock. Neutrons made by muons passing through high-Z materials around the detector.

43. COUPP: Chicagoland Observatory for Underground Particle Physics (FNAL Test Beam Program T-945) J. Collar, D. Nakazawa, B. Odom, A. Sonnenschein, K. O’Sullivan Kavli Institute for Cosmological Physics The University of Chicago P. Cooper, M. Crisler, J. Krider, K. Krempetz, C.M. Lei, H. Nguyen, E. Ramberg, R. Schmitt, A. Sonnenschein, R. Tesarek Fermi National Accelerator Laboratory

44. Potential Sensitivity Of 1-Liter Chamber at Fermilab Site Spin-independent Spin-dependent Goal for this phase: reduce background to <1 event per liter per day

45. Design Concept for Large Chambers Central design issue is how to avoid metal contact with superheated liquid. Fabrication of large quartz or glass pressure vessels is not practical, but industrial capability exists for thin-walled vessels up to ~ 1 m 3 in volume. Thin- walled quartz bell jar Steel pressure vessel Pressure balancing bellows Buffer fluid

46. Installation of 1 Liter Chamber At Fermilab NuMi Tunnel 100 times active mass of test tube chamber. Prototypes many design features required for chambers with volume ~1000 liters

47. Neutron Shielding and Muon Veto Detector is surrounded by 30 cm of polyethylene neutron moderator. Very effective for low energy neutrons coming from ( ,n) radioactivity in rock. Simulations show reduction in rate to < 1/month. Active muon veto: 150 plastic scintillator counters from KTEV. Goal: 98% efficiency. To be installed in spring ‘06.

48. Movie: One of the First Events

49. 160 msec of Video Buffer (20 msec/frame)

50. Muon Track @ 160 psi Superheat Pressure

51. Subtracted image projection of subtracted image onto horizontal axis projection with noise subtraction Starting imageZoom In Image Analysis Procedure

52. Spatial Distribution of Single Bubbles, First 42 Days Duty cycle of chamber allowed 33% live time (14 live days)

53. Space & Time Distribution of Single Bubbles Rate in fiducial volume: 25.6 +/- 2.8 per live-day

54. Number of Bubbles Counts Multiple Bubble Analysis Statistics of multiple bubble events suggests 10% of bulk singles (2.8 /day) are due to neutrons. These should be coincident with cosmic rays-- we’ll see when Muon veto is installed.

55. Rate [Hz/Liter] Superheat Pressure [psi] Goal (1/ day) First NuMi data point surface basement (6 m.w.e.) Event Rate at NuMi Compared to Surface Rate

56. Conclusions We have demonstrated an operating mode for bubble chambers that in principle allows great sensitivity to WIMPs. Have a mechanical design scalable to ~ton target masses at low cost. Have built a fully working detector with active mass comparable to best competing experiments. Demonstrated insensitivity to gamma ray backgrounds at 10 -9 level. Neutron backgrounds are under control, less than 0.1 event/day seems achievable with cosmic veto counters in NuMi tunnel. Making progress on reducing alpha backgrounds.