Cosmic-ray electron spectrum

Last fall, the ATIC balloon collaboration reported a tantalizing peak near 500 GeV in its measured spectrum of high-energy cosmic-ray electrons (Physics Today, January 2009, page 16). The peak suggested that 500-GeV dark-matter particles of the kind predicted by extra-dimensional extensions of standard particle theory might be annihilating each other in nearby accumulations of dark matter to produce energetic electron–positron pairs. Now NASA’s recently launched Fermi Gamma-ray Space Telescope has measured the electron spectrum out to 1 TeV with much higher statistics (see the figure). Designed primarily to record high-energy gammas, Fermi can also detect electrons. The data show no narrow spectral feature near 500 GeV, nor anywhere else; but above 100 GeV there is a growing excess over the predictions of a conventional diffusive model of electrons from very distant astrophysical sources. The positron spectrum measured by the orbiting PAMELA magnetic spectrometer showed a similar...