In 2008, the Italian satellite PAMELA detected a curious excess of antimatter positrons – a startling discovery that could have been a sign of the existence of dark matter.
Now, in a scholarly paper submitted to Physical Review Letters, and originally published on the Internet physics archive, researchers with the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University are confirming the previous detection but with one important caveat; they didn't find a sudden drop-off of this excess in those cosmic rays beyond a certain energy level.
Many theories had predicted this would happen if dark matter was involved. The result casts doubt on the dark matter explanation for this excess.
Along with the findings being noteworthy, so is the way the researchers went about their work. To make their determination, the researchers – Stefan Funk and Justin Vandenbroucke – - relied on the Fermi Gamma-ray Telescope. The challenge: the telescope is designed for detecting neutral photons, and doesn't have a magnet for separating negatively charged electrons and positively charged positrons, which was essential for the experiment. The solution: the researchers, along with graduate student Warit Mitthumsiri, found a magnet a few hundred miles away from the telescope that did the trick. It was the Earth, which, thanks to its magnetic field, could bend the paths of charged particles raining more or less continuously from space.
After studying up on geophysics maps and calculating precisely how the Earth was filtering out charged particles seen by the telescope, the researchers went ahead with their analysis, and wound up with their dramatic results. "The big takeaway here is how valuable it is to measure and understand the world around us in as many ways as possible," says Vanenbroucke. "Once you have this basic scientific knowledge, it's often surprising how that knowledge can be useful."
"It was very satisfying that our approach, novel as it was, seemed to work so well," says Funk.
The complete interview with Funk and Vandenbroucke is available at: http://www.kavlifoundation.org/science-spotlights/KIPAC-earthmag-Funk-Vandenbroucke
Journal
Physical Review Letters