Fermi gamma-ray amplitude telescope confirms abstruse advantage of positrons

An inside view of SLAC's large detectors.

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That beforehand analysis by an apparatus alleged PAMELA had set off a access of speculation: Did the added positrons – the antimatter mates of electrons – appear from an astrophysical source, such as pulsars, or from a added alien agent – the abolishment of aphotic amount particles? Both sources accept their proponents. Pulsars are maelstroms of alluring armament that are still not calmly understood, and while aphotic amount particles are glace customers, through force aphotic amount has had a big effect, abstraction galaxies and influencing the anatomy of the universe.

The Fermi results, reported in a paper posted on arxiv website and submitted to the journal Physical Review Letters, doesn’t settle the question of where the extra positrons came from. But they represent an important confirmation of the earlier results, and extend the observation to more energetic positrons than before.

This confirmation of the PAMELA results “is extremely important whether it’s dark matter or not,” said Michael Peskin, a theoretical physicist at SLAC and a dark matter expert. Not everyone accepted the PAMELA results, according to Peskin: “There was some doubt the effect was real.”
While the positron debate percolated, the team working with Fermi’s main instrument, the Large Area Telescope, quietly started making it jump through hoops.

Since the LAT was designed to detect neutral photons – r, the highest-energy photons known in the universe – it doesn’t carry the magnet needed to separate negatively-charged electrons from positively-charged positrons, so the LAT team could count them.

“The Fermi satellite is not a perfect instrument to look for electrons and positrons,” said Stefan Funk of the joint SLAC/Stanford Kavli Institute for Particle Astrophysics and Cosmology, who led the team that analyzed the current results. The LAT was not designed to distinguish electrons and positrons, and this is hard to change, since the satellite is now in orbit 340 miles above the Earth.

The Earth’s magnetic field naturally bends the paths of charged particles approaching from space and the Earth’s bulk blocks the paths of positrons coming in from some directions, and electrons coming in from other directions. Combined, these two effects can be used to tell the LAT what regions of the sky to look in order to see only positrons or only electrons. “It’s essentially using the Earth’s magnetic field as a particle selector,” Funk said, “and the Earth itself as a shutter – ” a simple concept, but, added Funk, not simple in execution. He especially commended graduate student Warit Mitthumsiri and KIPAC post-doctoral researcher Justin Vandenbroucke as deserving of kudos for their efforts.

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