An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV

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Abstract
Cosmic ray positrons are known to be produced in interactions in the interstellar medium. As well as originating from this 'secondary source', positrons might also be generated in primary sources such as pulsars and microquasars — or by dark matter annihilation. A new measurement of the positron fraction in the cosmic radiation for the energy range 1.5–100 GeV has been made using data from the PAMELA satellite experiment. Previous measurements, made predominantly by balloon-borne instruments, yield a positron fraction compatible with 'secondary source' production from interactions between cosmic ray nuclei and interstellar matter. Above 10 GeV the new measurements deviate significantly from this expectation, pointing to the presence of a primary source, either a nearby astrophysical object or dark matter particle annihilations. Cosmic ray positrons are known to be produced by interactions in the interstellar medium, but they might also originate in primary sources, such as pulsars, micro-quasars or through dark matter annihilation. Adriani et al. report that the positron fraction increases sharply over much of the energy range 1.5–100 GeV, which appears to be completely inconsistent with secondary sources—they therefore conclude that a primary source is necessary. Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium1, which is referred to as a ‘secondary source’. Positrons might also originate in objects such as pulsars2 and microquasars3 or through dark matter annihilation4, which would be ‘primary sources’. Previous statistically limited measurements5,6,7 of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5–100 GeV. We find that the positron fraction increases sharply over much of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.
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