Positrons – read more

Figure 1 shows the earliest PAMELA results that for the first time have clearly revealed a significant increasing of the positron to all electron ratio above 10 GeV along the results of previous experiments. They are compared with a theoretical calculation that shows the effect of a pure secondary production of positrons during the propagation of cosmic rays in the Galaxy. The standard theory suggests that positron fraction is expected to fall as a smooth function of increasing energy if secondary production dominates. 

Figure 2 shows the final results of the positron fraction measured by PAMELA in the energy range 0.5 – 300 GeV.

Two features are clearly visible in the PAMELA data. At low energies, below 5 GeV, the positron fraction is 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 significant increasing 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 the leptonic component that shows an increasing in positron fraction and the hadronic component that does not present features or structures expected from exotic sources in the antiproton-proton ratio, difficult to explain in the framework in which the neutralino is the dominant dark matter component. A suitable explanation was obtained in terms of direct leptonic annihilation, e+, e, μ+, μ– channels 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+ – e– couples and accelerate them in their magnetosphere. Other models consider this excess of positrons as due to a standard production in some inhomogeneity in the SuperNovae Remants in our Galaxy or in the same site where protons are accelerated.

At low energy, the differences with previous data and calculation are due to the different activity period of the Sun in which measurements have been performed or, respectively, considered.

Figure 3 shows the measured positron spectrum by PAMELA in the energy region 1.5 – 300 GeV.  It is worth to note a flattening in the flux between 7 – 30 GeV, followed by a clear increasing.