Does a Shower of Subatomic Positrons Mean We've Found Dark Matter?

By Eliza Strickland | April 2, 2009 9:02 am

PamelaThe mysterious stuff known as dark matter may have left a calling card at the edge of the Earth’s atmosphere where a space-faring satellite named PAMELA could pick it up. Researchers are reporting that PAMELA detected a high number of the subatomic particles called positrons, the positively-charged counterpoints to electrons, which could have been created by collisions between dark matter particles. “PAMELA found a number of positrons much higher than expected,” the mission’s principal investigator Piergiorgio Picozza [said]. “Many think this could be a signal from dark matter” []. But of course, others think there’s a more mundane explanation.

Dark matter is one of the greatest enigmas in astrophysics: It cannot be observed directly, so researchers have to study its effects on normal matter to try to deduce what it’s made of. The top candidates for dark matter, the heavy but invisible stuff that makes up 23 percent of the universe, are weakly-interacting massive particles. Contrary to their WIMPy name, when two of these particles collide, they annihilate each other in a burst of energy and propel a cloud of matter and antimatter particles into space. The theory has been a favorite of physicists for years, but until now, no one had detected evidence of these collisions [Wired].

The new study, published in Nature, describes the PAMELA satellite’s investigations of the cosmic rays that constantly bombard our planet. Cosmic rays are actually particles, accelerated by supernovae remnants, then knocked around in a game of stellar pinball. They ultimately slam into the Earth’s atmosphere…. The rays are made up of various atomic and subatomic particles, and we detect them by observing the cascade of particles that are created when one hits our atmosphere [Ars Technica]. PAMELA found an unexpected amount of high-energy positrons, and say that there are only two likely sources for these particles: dark matter collisions, or the dense, spinning stars known as pulsars that emit beams of radiation.

While most physicists agree that the new findings are exciting, many are not convinced that dark matter’s signature has finally been detected, and are hoping further studies will clarify the positrons’ source. Nasa’s Fermi Gamma-ray Space Telescope, which was launched in June 2008, is already taking measurements from pulsars and should produce data that could clarify the mysterious signal. [Astrophysicist Nigel] Smith thinks pulsars provide the most likely explanation. “It’s the simplest solution,” he said. “I think everyone will be waiting for the Fermi data to come in” [BBC News].

Related Content:
80beats: Have Researchers Found Dark Matter’s Signature Over Antarctica?
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DISCOVER: The Father of Dark Matter Still Gets No Respect
DISCOVER: A (Dark) Matter of Time explores creative approaches to finding the stuff

Image: PAMELA Collaboration

CATEGORIZED UNDER: Physics & Math, Space
  • Uncle Al

    If dark matter exists, is electrically neutral, and if it sources a positron eccess, what balances the charge?

    Supernova ejecta contain Ni-56 then Co-56. Co-56 decays by positron emission, Astrophys. J. 673 1014 (2008), Chin. J. Astron. Astrophys. 8 71 (2008). “Type I supernova is a prolific event for 56Ni nucleosynthesis. Each one ejects almost half of a solar mass of 56Ni”, Handbook of Isotopes in the Cosmos: Hydrogen to Gallium, Donald D. Clayton, 2003. Why is positrons/electrons greater than one a surprise?

  • Ryan

    Uncle Al, my guess would be that these dark matter anhilations also produce an equal number of high energy electrons. However, these electrons would not be distinguishable from other electrons that come from well known local sources. Consequently, I think the electron side of the dark matters’ charge is just assumed here.

  • John Cassady

    Jeez I’m so glad that you two are here otherwise we’d only have the experts to listen to.

  • Julius Mazzarella

    Uncle Al and Ryan. Thank you. I enjoyed reading your comments. ! I’m glad to see there are others interested in this topic and willing to share their knowledge.

    Have a great day.


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