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The electrons constitute 1% of the cosmic rays. Evidence for synchrotron X-ray emission strongly supports the hypothesis that supernovae are the sources of the primary galactic electrons. A small fraction of electrons, of the order of 10%, are secondary and arise out from the interaction of cosmic rays, mostly protons and helium nuclei, on the interstellar matter. They are the end product of the decay of short-lived particles (mostly pions via the decay π±→μ±e±) produced in these interactions.
Figure 1  shows the electron energy spectrum measured by PAMELA between 1 GeV and 625 GeV, along with other recent experimental data. It is described by a single power law energy dependence, with spectral index -3.18 ± 0.05 above the energy region in influenced by the solar wind (> 30 GeV). It is interesting to note, in Figure 2 , that both PAMELA and FERMI spectra harden and hint to a structure around 100 GeV, not unlike a similar feature noticeable in the ATIC all-electron spectrum reported in Figure 3 . Such hardening could be explained by additional leptonic components with a hard spectrum which, contributing equally to electrons and positrons, would likewise explain the increase in the positron fraction measured by PAMELA and FERMI (see Figure 4  as an example).


Edited by Vincenzo Buttaro