PAMELA may have spotted the dark stuff

Data from an orbiting observatory record more positrons than standard model accounts for

Normal 0 false false false MicrosoftInternetExplorer4 Normal 0 false false false MicrosoftInternetExplorer4 Cosmologists are agog about the possibility that an orbiting observatory may have discovered particles of dark matter — the proposed, 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 that the device had recorded more positrons from the Milky Way than could be accounted for by the standard model of elementary particle physics.

At the International Conference on High Energy Physics, held in Philadelphia, PAMELA researcher Mirko Boezio of the Italian National Institute of Nuclear Physics in Trieste suggested that the surplus of positrons — the electron’s antiparticle, which is equal in mass but opposite in charge — could be accounted for by the annihilation of pairs of dark-matter particles. According to an existing theory, when dark-matter particles collide, they decay into a spray of ordinary, visible particles, including an abundance of 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, the findings remain preliminary, and “We prefer to withhold further comments.”

But that hasn’t stopped other researchers from posting their interpretations of the data on the Internet.

In a paper posted August 28, Marco Cirelli of the Institute of Physical Theory at the French Atomic Energy Commission near Paris and Alessandro Strumia of the University of Pisa and the Italian nuclear physics institute report that the PAMELA findings appear to be consistent with the existence of a proposed dark-matter particle known as minimal dark matter (

Their minimal model would introduce a set of five new elementary particles that interact only through a weak nuclear force. The theory suggests that a dark-matter particle is about 10,000 times heavier than a proton.

The model “adds to the standard model of particle physics the minimal amount of new ingredients that allows [researchers] to explain the existence and the properties of dark matter without ruining the good features of the standard model,” Cirelli and Strumia wrote in an e-mail message.

Astronomers have proposed the existence of dark matter for decades because it would provide the unseen glue that keeps galaxies intact and galaxy clusters from disassembling. However, dark matter has never been convincingly detected.

Although the preliminary PAMELA data are intriguing, the researchers caution that “There might still be adjustments or calibrations that will change the data, and this is only in the hands of the PAMELA collaboration.” Astronomers must also make sure that ordinary astrophysical objects that produce positrons, such as supernovas, are not responsible for the excess particles.

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. That rise is just what the annihilation of dark-matter particles predicts. In contrast, known astrophysical processes would produce a more gradual increase in positrons.

The upturn “is a classic signature of new physics,” says Vernon Barger of the University of Wisconsin–Madison. He and his collaborators calculate that the annihilation of cold dark-matter particles can explain the excess of positrons detected by the observatory ( Cold dark-matter particles move slowly compared with the speed of light and are most widely used in cosmological theories, much more so than minimal dark matter particles.

In another recently posted paper, University of Stockholm researchers Lars Bergström, Torsten Bringmann and Joakim Edsjö suggest that PAMELA may have detected a type of dark matter predicted by supersymmetry, a proposed extension of the standard particle physics model that would double the number of known elementary particles ( The lightest of these supersymmetric particles, which would have been forged immediately after the Big Bang, would be stable and could exist today, the researchers note. Moreover, a small fraction of these proposed dark-matter particles would encounter each other in space. Such meetings would end in destruction because each of these particles acts as its own antiparticle. And when the particles annihilate, they produce an assortment of ordinary particles, including positrons.

But Bergström and Bringmann caution that any model that invokes dark matter to account for the positron excess still requires fine-tuning to match the PAMELA data. For instance, their own model requires a thousand-fold boost in the predicted rate that dark-matter particles in the Milky Way, on average, would annihilate. This increase could be achieved if some parts of the galaxy harbor unusually dense clumps of dark matter, they suggest.

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