The cosmic ray detector aboard the PAMELA spacecraft. Illustration: Courtesy of the PAMELA Collaboration.
Last August, a stir went through the physics community when a presenter at the International Conference on High Energy Physics in Philadelphia, Pennsylvania, showed some data from the PAMELA satellite. The data indicated a surprising excess of positrons and anti-protons in the cosmic rays raining down on Earth. Various commenters and scientists speculated that this could be a signal of dark matter particles annihilating.
The incident became more intriguing when other scientists used photos, taken during the presentation, of the slides to extract the data and then present analysis of that data in other scientific papers. (See "Paparazzi or preprints?") Unresolved at the time was whether these signals really were evidence for dark matter.
A new paper published in Physical Review Letters this week, presents data from 500 days of data collection by PAMELA, and an analysis of the ~1000 anti-protons observed, including ~100 very high energy anti-protons. The collaboration concludes that the data is not likely to be a sign of dark matter annihilation, but places some very stringent constraints on how much of a signal of dark matter could be present in the cosmic ray flux.
An excellent (but somewhat technical in places) commentary and explanation of these results appears in Physics and is well worth digging in to if you are interested in the topic, but aren't sure if you are up to reading the full paper.
Update: Sean Carroll at Cosmic Variance has a bunch more very useful context and information on this work.
