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Positrons - Read More Print
Figure 1 shows the last results of the positron to all electron ratio measured by the PAMELA experiment in the energy range 0.5 - 300 GeV (to be published). Figure 2  shows the already published data along with other recent experimental results. The calculation, in the same gure, shows the eff ect of a pure secondary production of positrons during the propagation of cosmic rays in the Galaxy, without reacceleration processes. It suggests that positron fraction is expected to fall as a smooth function of increasing energy if secondary production dominates. 
Two features are clearly visible in the PAMELA data. At low energies, below 5 GeV, they are systematically lower than data collected during the 1990s, between 5 GeV and 10 GeV they are compatible with other measurements, while at high energies, above 10 GeV, they show a positron fraction increasing signi ficantly with the energy. This interesting excess of positrons in the range 10-300 GeV has led to many speculations about its origin, as annihilation of dark matter, decaying dark matter, cosmic strings, young pulsars, a few nearby SNR. In a supersymmetric scenario, the PAMELA results set an intriguing theoretical challenge because of the asymmetry between leptonic (positron fraction) and hadronic (antiproton-proton ratio), difficult to explain in the framework in which the neutralino is the dominant dark matter component. A suitable explanation is obtained in terms of direct leptonic annihilation, e+, e-, μ+, μchannels for a wide range of the WIMP mass. Furthermore, explanations in term of dark matter annihilation request a boost factor for the annihilation standard rate. 
Another interpretation considers a contribution from nearby and young pulsars, objects capable to produce e+ ecouples and accelerate them in their magnetosphere.
At low energy, the di fferences with previous data are due to the di fferent activity period of the Sun in which measurements have been performed.


Edited by Laura Marcelli