Bubble Chambers: Old Tools In New Searches For Dark Matter Geoffrey Iwata Physics /16/10

The Bubble Chamber: A brief history Cloud chambers were not good enough. Donald Glaser Was dissatisfied in his search for strange particles with cloud chamber. Bubble chamber concept in Michigan Was not inspired by beer Conceptually similar to a cloud chamber, but uses a superheated liquid, as opposed to a supercooled gas.

Bubble Chambers: How do they work?

Metastable States For water to boil, bubbles of water vapor grow, rise to the surface, and burst. But if they start out too small, surface tension will suppress growth. Like a balloon- hardest to begin filling. Surface tension: Δp α 1/R

How do we make Metastable states? Liquid is in a chamber kept just below its boiling point. Pressure is quickly reduced with a piston to lower the boiling point of the liquid below the temperature of the liquid, leaving it in a superheated state. You can also do it in your microwave!

The Bubble Chamber Charged particles passing through will: ionize the atoms  vaporize the liquid  create microscopic bubbles  which will expand.

What do bubble chambers look like?

What do bubble Chambers have to do with Dark Matter?

Searching for WIMPS Weakly Interacting Massive ParticleS Cold, large-mass particles, that interact only via Weak and Gravitational forces. Neutral, and 100x heavier than proton. Since slow moving, would tend to clump together, providing basis for cold dark matter model.

Searching for WIMPS Current model predicts spherical halo of neutralinos (WIMPS) in our own galaxy. Halo particle density should fall off with distance from center of galaxy as 1/r 2. At Earth, density should be around 0.3 GeV/cm 3. Velocity distribution of the halo WIMPS is a bell shape curve with a dispersion (variance) of v=230 km/s. Must also add the relative velocity of the solar system, 244 km/s, with respect to the halo Gives WIMP distribution with a mean velocity of 270 km/s Artist rendition

How do Bubble Chambers help? Metastable states can be “tuned” to certain energies. At a given temperature, bubble formation on the track occurs, if, within a region of critical size l crit, the deposited energy, E dep, exceeds a threshold energy E min High background rejection!

How do Bubble Chambers help? For example, if tuned to threshold of 5 KeV, gamma-ray induced events are rejected by more than a factor of ten million! This is ideal for dark matter experiments.

Experiments We will focus on two main experiments: PICASSO COUPP

The Picasso Experiment Project In CAnada to Search for Supersymmtric Objects A large droplet detector surrounded by water for high background rejection. Measures Spin-dependent neutralino interactions C4F10 Picasso operates between 20 and 47 degrees.

The Droplet Detector Superheated droplets suspended in viscous gel. Mixture has usually ~1% superheated liquid Usually use Freons, such as CCl2F2, C2ClF5, C4F10, C3F8 Emulsion can be stable for months at atmospheric pressure.

The Picasso Experiment Millions of 100 μm size droplets in superheated C4F10. Records bubble forming events with acoustic detectors, then triangulates.

The Picasso Experiment July, 2009 – No dark matter yet. But: New limits on 24 Gev/c^2 WIMP scattering cross section of 13.5 pb on F. Converted to 0.16 pb for proton.

COUPP (Chicagoland Observatory for Underground Particle Physics) Heavy liquid (Trifluoroiodomethane (CF3I)) filled bubble chamber. Also has high background rejection capabilities: predicted 1 WIMP event per year Not as superheated as conventional bubble chambers to reject minimally ionizing events. Chamber recompresses after each event.

COUPP (Chicagoland Observatory for Underground Particle Physics)

COUPP Sensitivity Recoil must be over thresholds in both E and dE/dx.

COUPP (Chicagoland Observatory for Underground Particle Physics) Multiple scattering vs single WIMP event.

COUPP (Chicagoland Observatory for Underground Particle Physics)

COUPP Results: Acoustic parameter describes acoustic energy deposited in event.

COUPP Results Experiment has failed to find any Dark Matter particles. But, like Picasso, has put new limits on spin- dependent WIMP scattering cross sections.

COUPP Results: Spin-dependent interactions

COUPP Results Experiment has failed to find any Dark Matter particles. But, like Picasso, has put new limits on spin- dependent WIMP scattering cross sections. In disagreement with DAMA results. DAMA experiment cannot be explained by spin- independent interactions.

COUPP Results "It is impossible to make a direct comparison between the COUPP and DAMA results. In particular, COUPP uses different target materials and approaches [to DAMA].” –Rita Bernabei, University of Rome

In Conclusion Bubble chambers, while old in technology, are still valid tools for modern experiments Dark Matter remains pretty elusive. The search continues!

References a/dm.php /39/1/027/pdf/jpconf6_39_027.pdf theory.fnal.gov/Conferences/TeV/Sonnenschein.pdf Wikipedia.com