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Pamela spots the dark stuff, maybe: orbiting observatory records excessive positron production.

Cosmologists are agog about the possibility that an orbiting observatory may have discovered particles of dark matter--the invisible material that researchers believe makes up most of the mass of the universe.

At two meetings in August, researchers analyzing data from the Russian-European observatory PAMELA, short for Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics, reported preliminary evidence for the detection of more positrons from the Milky Way than could be accounted for by the standard model of particle physics.

At the International Conference on High Energy Physics, held in Philadelphia, Mirko Boezio of the Italian National Institute of Nuclear Physics in Trieste suggested that the surplus of positrons--the electron's antiparticle--could be accounted for by the annihilation of pairs of dark matter particles. When dark matter particles collide, one theory predicts, they decay into a spray of ordinary particles, including positrons.

"We plan to have final results ready by early October and submit a paper to a peer-reviewed journal," Boezio told Science News. Until then, he says, "We prefer to withhold further comments." But that hasn't stopped other researchers from posting their interpretations of the data on the Internet.

One report suggests that the PAMELA findings are consistent with the existence of a proposed dark matter particle known as minimal dark matter.

The model that predicts that particle would introduce a set of five new elementary particles that interact only through the weak nuclear force, wrote Marco Cirelli of the Institute of Physical Theory of the French Atomic Energy Commission and Alessandro Strumia of the University of Pisa and Italy's nuclear physics institute in a paper posted August 28 (arxiv. org/abs/0808.3867). Their theory suggests that a dark matter particle is about 10,000 times heavier than a proton.

One promising feature of the PAMELA data, Cirelli and Strumia say, is that the observatory appears to have recorded an abrupt rise in positrons with energies of about 10 billion electronvolts--just what the annihilation of dark matter particles predicts. In contrast, known astrophysical processes would produce a more gradual increase in positrons.

In another recently posted paper, Lars Bergstrom, Torsten Bringmann and Joakim Edsjo of Stockholm University suggest that PAMELA may have found evidence of a type of dark matter predicted by supersymmetry, an extension of the standard particle physics model that would double the number of elementary particles (arxiv.org/abs/0808.3725).

Astronomers infer the existence of dark matter because it would provide the unseen glue that keeps galaxies intact and galaxy clusters from disassembling. But dark matter has never been convincingly detected directly.
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Title Annotation:Atom & Cosmos
Author:Cowen, Ron
Publication:Science News
Geographic Code:1USA
Date:Sep 27, 2008
Words:426
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