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Based on general considerations rather than model-dependent fits to specificscenarios, we argue that an increase with energy of the positron fraction incosmic rays, suggested by several experiments at E>~7 GeV, most likely requiresa primary source of electron-positron pairs. We discuss the possiblealternatives, and find none of them plausible on astrophysical or particlephysics grounds. Further observational ways to test different scenarios arediscussed.

Possible causes of a rise with energy of the cosmic ray positron fraction