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

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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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Figure 1: Calorimeter energy fraction, .
Figure 2: PAMELA positron fraction with other experimental data and with secondary production model.


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We thank D. Marinucci for discussions concerning statistical methods, D. Müller, S. Swordy and their group at University of Chicago, G. Bellettini and G. Chiarelli for discussions about the data analysis and L. Bergström for comments on the interpretation of our results. We acknowledge support from The Italian Space Agency (ASI), Deutsches Zentrum für Luftund Raumfahrt (DLR), The Swedish National Space Board, The Swedish Research Council, The Russian Space Agency (Roscosmos) and The Russian Foundation for Basic Research.

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Correspondence to P. Picozza.

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