Fermi Identifies Cosmic Antiparticles in the Shadow of the Earth

May 5, 2015

Although the Fermi Gamma-ray Space Telescope is primarily a gamma-ray instrument, its most cited paper reports a measurement of the combined electron and positron cosmic-ray spectrum. Now a team, led by KIPAC researchers, has built on this result by using a novel technique to separate the cosmic-ray electrons and positrons and measure the spectrum of each component individually. The result will keep theorists busy thinking about pulsars and dark matter.

Fraction of positrons among positron and electron cosmic rays, as a function of energy, as seen by Fermi, PAMELA, and other instruments.

In the last three years there has been renewed excitement about cosmic-ray electrons and positrons. In 2009 the PAMELA satellite reported that the fraction of positrons - that is, the ratio of the positron flux to the combined electron-plus-positron flux - increases with energy above ~10 GeV. This indicates an unexplained excess of cosmic-ray positrons above the standard prediction that they are produced by collisions of accelerated atomic nuclei with interstellar gas. This "secondary production" model predicts a fraction of positrons that decreases with energy. Many theories have been proposed to explain the excess positrons, including nearby pulsars and annihilating dark matter.
In addition to detecting the gamma rays it was designed for, the Fermi Gamma-ray Space Telescope's Large Area Telescope (LAT), which was built at and is operated from SLAC, can detect the fast high energy charged particles that comprise cosmic rays. Cosmic-ray experiments on balloons and satellites typically use a magnet to separate the positively and negatively charged particles by bending them. But because the LAT was designed to detect gamma ray photons, it has no magnet. Instead, the LAT team exploited the Earth’s magnetic field and the shadow of the Earth to distinguish positrons and electrons. The trajectories of individual particles from certain directions are blocked by the Earth, and because of the way the Earth’s magnetic field bends charged particles, these shadows are offset in different directions for positively and negatively charged cosmic rays.
The result of the analysis is a new measurement of the electron-only and positron-only spectra between 20 and 200 GeV. The new measurements from Fermi agree with those from PAMELA, confirming this important discovery that the positron fraction increases with energy. KIPAC Professor Roger Romani proposed the technique to use the Earth’s magnetic field and relative position to separate electrons and positrons with LAT observations, and KIPAC Professor Stefan Funk, postdoctoral scientists Justin Vandenbroucke, and graduate student Warit Mitthumsiri led the analysis, along with Carmelo Sgro of the Italian Instituto Nazionale di Fisica Nucleare in Pisa, and former KIPAC postdoc Marcus Ackermann, now with Deutches Elektronen-Synchrotron in Germany.
With this confirmation of the seemingly anomalous cosmic ray positron excess at high energies, a major priority for the astrophysics community will be explaining the phenomenon with a new astrophysical or dark matter process.  The AMS-02 experiment, operating on the international space station since May, is expected to improve these measurements significantly, with better statistics and energy reach. Fermi has yet again shown that the most exciting results from any given instrument are sometimes the ones that were never even anticipated.
This work is reported in a paper submitted to Physical Review Letters and available at arXiv:1109.0521.
Science contact:
Justin Vandenbrouke
Tidbit Author: Justin Vandenbrouke and Jack Singal