Transcript Bubble Chambers: Looking for Dark Matter (and trying to

Bubble Chambers:
Old Tools In New Searches For Dark Matter
Geoffrey Iwata
Physics 129
11/16/10
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.
Artist rendition
• 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
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)
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 spindependent 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 spindependent WIMP scattering cross sections.
•In disagreement with DAMA results.
•DAMA experiment cannot be explained by spinindependent 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
• http://www.picassoexperiment.ca/dm.php
• http://cerncourier.com/cws/article/cern/29120ent.c
a/dm.php
• http://iopscience.iop.org/17426596/39/1/027/pdf/jpconf6_39_027.pdf
• http://www-astrotheory.fnal.gov/Conferences/TeV/Sonnenschein.pdf
• http://news.uchicago.edu/news.php?asset_id=2063
• Wikipedia.com