An Alternative Approach to Understanding the Observed Positron FractionReport as inadecuate

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Brazilian Journal of Physics

, Volume 44, Issue 5, pp 530–533

First Online: 25 June 2014Received: 28 April 2014


Space-based observations by PAMELA Adriani et al., Nature 458 , 607, 2009, Fermi-LAT Ackerman et al., Phys. Rev. Lett. 105 , 01103, 2012, and AMS Aguilar et al., Phys. Rev. Lett. 110 , 141102, 2013 have demonstrated that the positron fraction e+-total-e increases with increasing energy above about 10 GeV. According to the propagation model for Galactic cosmic rays in widespread use Moskalenko and Strong, Astrophys. J. 493 , 693, 1998, the production of secondary positrons from interaction of cosmic-ray protons and heavier nuclei with the interstellar medium gives a generally falling positron fraction between 10 and 100 GeV, with secondary positrons accounting for only ∼20 % of the observed positron fraction at 100 GeV; so some other physical phenomena have been proposed to explain the data. An alternative approach to interpreting the positron observations is to consider these data as presenting an opportunity for re-examining models of Galactic cosmic-ray propagation. Following release of the PAMELA data, three groups published propagation models Shaviv, et al., Phys. Rev. Lett. 103 , 111302, 2009, Cowsik and Burch, Phys. Rev. D. 82 , 023009, 2010, Katz et al., Mon. Not. R. Aston. Soc. 405 , 1458 2010 in which the observed positron fraction is explained entirely by secondary positrons produced in the interstellar medium. In May of this year, stimulated by the AMS extension of the positron data to higher energy with excellent statistics, two of those groups presented further development of their calculations Cowsik et al. 2013, Blum et al. 2013, again concluding that the observed positrons can be understood as secondaries. None of the authors of these five papers was registered for the 33rd International Cosmic Ray Conference ICRC. Although I am not an author of any of these papers, I have some close familiarity with one of these recent papers, so the conference organizers invited me to bring this alternative approach to the attention of the conference. The present paper is a summary of the material I presented, along with a brief comment about reaction at the conference to this approach.

KeywordsPositron fraction Cosmic-ray positrons Galactic cosmic-ray propagation  Download fulltext PDF

Author: Martin H. Israel


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