Re: Bubble chamber puts new constraints on WIMPs

On 19 fév, 12:14, Sam Wormley <sworml...@xxxxxxxxx> wrote:
Bubble chamber puts new constraints on WIMPs

Bubble chambers, which were first used in the 1950s to detect
electrically charged particles, might sound as if they should belong
firmly to particle-physics history books. Now, however, physicists
working at the Chicagoland Observatory for Underground Particle
Physics (COUPP) experiment in the US have resurrected the technique
to search for dark matter. Although their bubble-chamber experiments
have failed to find any dark-matter particles, the null result has
imposed new limits on certain properties of weakly interacting
massive particles (WIMPs), which are a leading candidate for dark

The COUPP team also says that their results casts further doubt on
claims made in 1998 by members of the Dark Matter (DAMA) experiment
at the Gran Sasso National Laboratory in Italy to have observed
WIMPs. The DAMA team -- which used a large array of sodium-iodide
detectors located 1400 m below ground -- insists that it is
impossible to make a direct comparison between the two experiments.

Invisible to telescopes

Dark matter was first proposed about 70 years ago to explain the
abnormally high rotation speeds of galaxies, which would otherwise be
torn apart if they did not contain vast amount of hidden mass
providing extra "gravitational glue". It is fundamentally different
from normal "luminous" matter such as stars as it is invisible to
modern telescopes, giving off no light or heat, and seems to interact
only through gravity.

Physicists believe that WIMPs might interact with ordinary matter
through several different mechanisms that are either dependent or
independent of the nuclear spin of atoms in ordinary matter. Although
various experiments appear to have ruled out the possibility that the
DAMA observations came from spin-independent interactions, they could
still have been explained by a spin-dependent mechanism. Now that
possibility appears to have been ruled out as well by the latest
COUPP results.

Super-heated state

The COUPP experiment, which is located some 100 m underground in a
tunnel at Fermilab, consists a glass jar filled with about a litre of
CF3I - a liquid that is normally used as a fire-extinguisher (Science
319 933). The liquid is heated to just below its boiling point, at
which point a piston is withdrawn ,which makes the chamber expand.
This reduces the pressure of the fluid, leaving it in a super-heated

If a WIMP collides with a nucleus in the fluid, the recoiling nucleus
heats the surrounding fluid, which boils and forms a tiny bubble. The
bubble gets bigger as the chamber expands until it is large enough -
about a millimetre in size - to be photographed by digital cameras.

COUPP watches for WIMP collisions nearly all of the time. By doing a
statistical analysis of the bubbles in a large number photographs,
the team says that they can work out which were caused by dark matter
and which were caused by background radiation such as the alpha
particles emitted during the radioactive decay of radon. Unlike old
bubble chambers and most modern dark-matter search techniques, COUPP
is insensitive to most other kinds of background radiation such as
muons, gamma rays and X-rays.

This is baloney.

All bubble chambers, by very principle, always could detect muon
tracks simply because they are charged particles. On the other hand,
there is no way that gamma or x rays will leave a trace in any type
of bubble chamber since they are not charged.

You can give a hint to this Fermilab team that if they ever hope to
detect even one WIMP with a bubble chamber, it better be charged.

André Michaud

If the DAMA result had been due to spin-dependent WIMPs, then COUPP
researchers should have found hundreds of WIMPs. Instead, they found
none above background. "These results establish the bubble chamber as
a new competitive technique in the search for WIMPs as candidates for
dark matter," explains Fermilab's Peter Cooper who is a senior
scientist on COUUP. "They also contradict the observation claimed by
DAMA in the last region not already excluded by other dark matter
searches", he said.

DAMA disagrees

However, Rita Bernabei at the University of Rome -- spokesperson for
the DAMA experiment -- disagrees with that this conclusion. "It is
impossible to make a direct comparison between the COUPP and DAMA
results," she told "In particular, COUPP uses
different target materials and approaches [to DAMA]. She also pointed
out that COUPP addresses also just one of several possible models of
spin-dependent interaction, whereas the DAMA result was not tied to
any specific interaction model -- spin-dependent or otherwise.

COUPP is not the first experiment to put the DAMA results in question
and several dark-matter searches since 1998 have failed to turn up
any particles. However, Bernabei believes that like COUPP, these
experiments use targets and approaches that are different to DAMA and
therefore do not contradict their finding.

Spin-independent interactions

While COUPP does an impressive job of rejecting background radiation,
it is relatively insensitive to spin-independent interactions, which
are believed to be much more prevalent than spin-dependent
scattering. As a result, some physicists have suggested that it is
not a significant step towards the detection of WIMPs -- unlike
competing contemporary experiments such as XENON10 in Italy; CDMS in
the US; and ZEPLIN-II in the UK.

Cooper and his COUPP colleagues are now working to increase the
sensitivity by increasing the amount of liquid to 30 l in the bubble
chamber, and hope to start testing the larger chamber at Fermilab

"The path to much larger bubble chambers is clear and underway now,"
says Cooper. "We plan to be operating up to 100 Kg of sensitive mass
within a year. The ultimate limitations to this technique will be
only the ability to control backgrounds."