Results 1 to 2 of 2
  1. #1's Avatar Guest

    Default COUPP experiment tightens limits on dark matter

    Scientists working on the COUPP experiment at the Department of
    Energy's Fermi National Accelerator Laboratory announced a new
    development in the quest to observe dark matter.

  2. #2
    Sam Wormley's Avatar
    Sam Wormley Guest

    Default COUPP experiment tightens limits on dark matter 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.

    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."



Similar Threads

  1. Unravelling the Dark Matter - Dark Energy Paradigm
    By Surfer in forum UK Astronomy Forum
    Replies: 0
    Last Post: 03-28-2009, 02:06 PM
  2. COUPP experiment tightens limits on dark matter
    By in forum UK Astronomy Forum
    Replies: 0
    Last Post: 02-21-2008, 07:36 AM
  3. A Matter of Fact: NASA Finds Direct Proof of Dark Matter
    By Sam Wormley in forum Amateur Astronomy Forum
    Replies: 72
    Last Post: 08-27-2006, 10:20 PM
  4. Replies: 36
    Last Post: 10-16-2005, 03:13 AM
  5. Dark Matter & Dark Energy Two Aspects of the Same Thing?
    By Klaatu in forum Amateur Astronomy Forum
    Replies: 27
    Last Post: 07-12-2004, 11:32 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
Powered by vBulletin® Version 4.2.0
Powered by vBulletin®
All times are GMT. The time now is 08:13 AM.