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


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.


  1. Moskalenko, I. V. & Strong, A. W. Production and propagation of cosmic-ray positrons and electrons. Astrophys. J. 493, 694–707 (1998)

    ADS  CAS  Article  Google Scholar 

  2. Atoian, A. M., Aharonian, F. A. & Volk, H. J. Electrons and positrons in the galactic cosmic rays. Phys. Rev. D 52, 3265–3275 (1995)

    ADS  Article  Google Scholar 

  3. Heinz, S. & Sunyaev, R. Cosmic rays from microquasars: A narrow component in the CR spectrum. Astron. Astrophys. 390, 751–766 (2002)

    ADS  Article  Google Scholar 

  4. Jungman, G., Kamionkowski, M. & Griest, K. Supersymmetric dark matter. Phys. Rep. 267, 195–373 (1996)

    ADS  Article  Google Scholar 

  5. Golden, R. L. et al. Measurement of the positron to electron ratio in the cosmic rays above 5 GeV. Astrophys. J. 457, L103–L106 (1996)

    ADS  CAS  Article  Google Scholar 

  6. Barwick, S. W. et al. Measurements of the cosmic-ray positron fraction from 1 to 50 GeV. Astrophys. J. 482, L191–L194 (1997)

    ADS  CAS  Article  Google Scholar 

  7. Aguilar, M. et al. Cosmic-ray positron fraction measurement from 1 to 30 GeV with AMS-01. Phys. Lett. B 646, 145–154 (2007)

    ADS  CAS  Article  Google Scholar 

  8. Chang, J. et al. An excess of cosmic ray electrons at energies of 300–800 GeV. Nature 456, 362–365 (2008)

    ADS  CAS  Article  Google Scholar 

  9. Picozza, P. et al. PAMELA — A payload for antimatter matter exploration and light-nuclei astrophysics. Astropart. Phys. 27, 296–315 (2007)

    ADS  Article  Google Scholar 

  10. Delahaye, T. et al. Galactic secondary positron flux at the Earth. Preprint at 〈〉 (2008)

  11. Boezio, M. et al. The cosmic-ray electron and positron spectra measured at 1 AU during solar minimum activity. Astrophys. J. 532, 653–669 (2000)

    ADS  CAS  Article  Google Scholar 

  12. Alcaraz, J. et al. Leptons in near earth orbit. Phys. Lett. B 484, 10–22 (2000)

    ADS  CAS  Article  Google Scholar 

  13. Clem, J. & Evenson, P. in Proc. 30th Intl Cosmic Ray Conf. Vol. 1 (eds Caballero, R. et al.) 477–480 (Universidad Nacional Autónoma de México, 2008)

    Google Scholar 

  14. Aharonian, F. et al. First detection of a VHE gamma-ray spectral maximum from a cosmic source: HESS discovery of the Vela X nebula. Astron. Astrophys. 448, L43–L47 (2006)

    ADS  CAS  Article  Google Scholar 

  15. Berezhko, E. G., Ksenofontov, L. T. & Völk, H. J. Emission of SN 1006 produced by accelerated cosmic rays. Astron. Astrophys. 395, 943–953 (2002)

    ADS  CAS  Article  Google Scholar 

  16. Serpico, P. On the possible causes of a rise with energy of the cosmic ray positron fraction. Phys. Rev. D 79, 021302 (2009)

    ADS  Article  Google Scholar 

  17. Komatsu, E. et al. Five-year Wilkinson microwave anisotropy probe observations: Cosmological interpretation. Astrophys. J. Suppl. Ser. 180, 330–376 (2009)

    ADS  Article  Google Scholar 

  18. Servant, G. & Tait, T. M. P. Is the lightest Kaluza-Klein particle a viable dark matter candidate? Nucl. Phys. B 650, 391–419 (2003)

    ADS  Article  Google Scholar 

  19. Cheng, H. C., Feng, J. L. & Matchev, K. T. Kaluza-Klein dark matter. Phys. Rev. Lett. 89, 211301 (2002)

    ADS  Article  Google Scholar 

  20. Bertone, G., Hooper, D. & Silk, J. Particle dark matter: Evidence, candidates and constraints. Phys. Rep. 405, 279–390 (2005)

    ADS  CAS  Article  Google Scholar 

  21. Adriani, O. et al. A new measurement of the antiproton-to-proton flux ratio up to 100 GeV in the cosmic radiation. Phys. Rev. Lett. 102, 051101 (2009)

    ADS  CAS  Article  Google Scholar 

  22. Cholis, I., Dobler, G., Finkbeiner, D. P., Goodenough, L. & Weiner, N. The case for a 700+ GeV WIMP: Cosmic ray spectra from ATIC and PAMELA. Preprint at 〈〉 (2008)

  23. Bergström, L., Bringmann, T. & Edsjö, J. New positron spectral features from supersymmetric dark matter: A way to explain the PAMELA data? Phys. Rev. D 78, 103520 (2008)

    ADS  Article  Google Scholar 

  24. Donato, F., Maurin, D., Brun, P., Delahaye, T. & Salati, P. Constraints on WIMP dark matter from the high energy PAMELA data. Phys. Rev. Lett. 102, 071301 (2009)

    ADS  CAS  Article  Google Scholar 

  25. Grajek, P., Kane, G., Phalen, D. J., Pierce, A. & Watson, A. Is the PAMELA positron excess winos? Preprint at 〈〉 (2008)

  26. Grimani, C. Pulsar birthrate set by cosmic-ray positron observations. Astron. Astrophys. 418, 649–653 (2004)

    ADS  CAS  Article  Google Scholar 

  27. Büsching, I., de Jager, O. C., Potgieter, M. S. & Venter, C. A cosmic-ray positron anisotropy due to two middle-aged, nearby pulsars? Astrophys. J. 78, L39–L42 (2008)

    ADS  Article  Google Scholar 

  28. Yuksel, H., Kistler, M. D. & Stanev, T. TeV gamma rays from Geminga and the origin of the GeV positron excess. Preprint at 〈〉 (2008)

  29. Hooper, D., Blasi, P. & Serpico, P. D. Pulsars as the sources of high energy cosmic ray positrons. J. Cosmol. Astropart. Phys. 01, 025 (2009)

    ADS  Article  Google Scholar 

  30. Beatty, J. J. et al. New measurement of the cosmic-ray positron fraction from 5 to 15GeV. Phys. Rev. Lett. 93, 241102 (2004)

    ADS  CAS  Article  Google Scholar 

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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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Adriani, O., Barbarino, G., Bazilevskaya, G. et al. An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV. Nature 458, 607–609 (2009).

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