Bubble Chambers:Old Tools In New Searches For Dark Matter

Geoffrey IwataPhysics 12911/16/10

The Bubble Chamber:A brief history

• Cloud chambers were not good enough.• Donald Glaser-1952. 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/r2.

• At Earth, density should be around 0.3 GeV/cm3.• 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 lcrit, the deposited energy, Edep, exceeds a threshold energy Emin

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)

COUPPSensitivity

•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)

COUPPResults:

•Acoustic parameter describes acoustic energy deposited in event.

COUPPResults

• Experiment has failed to find any Dark Matter particles.

• But, like Picasso, has put new limits on spin-dependent WIMP scattering cross sections.

COUPPResults: Spin-dependent interactions

COUPPResults

• 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.

COUPPResults

• "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

• http://www.picassoexperiment.ca/dm.php• http://cerncourier.com/cws/article/cern/

29120ent.ca/dm.php• http://iopscience.iop.org/1742-6596/39/1/027/pdf/

jpconf6_39_027.pdf• http://www-astro-theory.fnal.gov/Conferences/TeV/

Sonnenschein.pdf• http://news.uchicago.edu/news.php?asset_id=2063• Wikipedia.com