Indications of proton-dominated cosmic-ray composition above 1.6 EeV

R. U. Abbasi, T. Abu-Zayyad, M. Al-Seady, M. Allen, J. F. Amman, R. J. Anderson, G. Archbold, K. Belov, J. W. Belz, D. R. Bergman, S. A. Blake, O. A. Brusova, G. W. Burt, C. Cannon, Z. Cao, W. Deng, Y. Fedorova, C. B. Finley, R. C. Gray, W. F. HanlonC. M. Hoffman, M. H. Holzscheiter, G. Hughes, P. Hüntemeyer, B. F. Jones, C. C.H. Jui, K. Kim, M. A. Kirn, E. C. Loh, J. Liu, J. P. Lundquist, M. M. Maestas, N. Manago, L. J. Marek, K. Martens, J. A.J. Matthews, J. N. Matthews, S. A. Moore, A. O'Neill, C. A. Painter, L. Perera, K. Reil, R. Riehle, M. Roberts, D. Rodriguez, N. Sasaki, S. R. Schnetzer, L. M. Scott, G. Sinnis, J. D. Smith, P. Sokolsky, C. Song, R. W. Springer, B. T. Stokes, S. Stratton, S. B. Thomas, J. R. Thomas, G. B. Thomson, D. Tupa, A. Zech, X. Zhang

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298 Scopus citations


We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (Xmax□), for air shower events collected by the High-Resolution Fly's Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate dxmax□/d[log□(E)] of 47.9±6.0(stat)±3.2(syst)g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4×1018eV.

Original languageEnglish (US)
Article number161101
JournalPhysical review letters
Issue number16
StatePublished - Apr 20 2010

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)

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