Published 2016
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Evidence for a mixed mass composition at the 'ankle' in the cosmic-ray spectrum

Creators

  • 1. University of Siegen
  • 2. University of Lisbon
  • 3. Fermilab
  • 4. University of São Paulo
  • 5. Ohio State University
  • 6. National Technological University
  • 7. National Autonomous University of Mexico
  • 8. University of Santiago de Compostela
  • 9. Complutense University of Madrid
  • 10. University of Bucharest
  • 11. Industrial University of Santander
  • 12. Centre national de la recherche scientifique
  • 13. Politehnica University of Bucharest
  • 14. Karlsruhe Institute of Technology
  • 15. University of Wuppertal
  • 16. University of Adelaide
  • 17. University of Grenoble
  • 18. Max Planck Society
  • 19. University of Paris
  • 20. Academy of Sciences of the Czech Republic
  • 21. University of Salento
  • 22. Federal University of Rio de Janeiro
  • 23. Polish Academy of Sciences
  • 24. Colorado State University
  • 25. RWTH Aachen University
  • 26. University of Granada
  • 27. Radboud University Nijmegen
  • 28. University of Catania
  • 29. INAF
  • 30. Universidad Michoacana de San Nicolás de Hidalgo
  • 31. State University of Campinas
  • 32. University of Naples Federico II
  • 33. Pennsylvania State University
  • 34. Case Western Reserve University
  • 35. University of Chicago
  • 36. University of Nantes
  • 37. University of Buenos Aires
  • 38. Federal Fluminense University
  • 39. University of Alcalá
  • 40. University of Rome Tor Vergata
  • 41. University of L'Aquila
  • 42. Federal University of Paraná
  • 43. National University of La Plata
  • 44. University of Hamburg
  • 45. ASTRON
  • 46. New York University
  • 47. Michigan Technological University
  • 48. University of Nova Gorica
  • 49. State University of Feira de Santana
  • 50. University of Groningen
  • 51. Colorado School of Mines
  • 52. Universidade Federal do ABC
  • 53. Benemérita Universidad Autónoma de Puebla
  • 54. University of Milan
  • 55. CINVESTAV
  • 56. Louisiana State University
  • 57. University of New Mexico
  • 58. Universidade Federal de Pelotas
  • 59. Northeastern University
  • 60. City University of New York
  • 61. Instituto Politécnico Nacional
  • 62. Vrije Universiteit Brussel
  • 63. University of Nebraska–Lincoln
  • 64. University of Leeds
  • 65. Los Alamos National Laboratory

Description

We report a first measurement for ultrahigh energy cosmic rays of the correlation between the depth of shower maximum and the signal in the water Cherenkov stations of air-showers registered simultaneously by the fluorescence and the surface detectors of the Pierre Auger Observatory. Such a correlation measurement is a unique feature of a hybrid air-shower observatory with sensitivity to both the electromagnetic and muonic components. It allows an accurate determination of the spread of primary masses in the cosmic-ray flux. Up till now, constraints on the spread of primary masses have been dominated by systematic uncertainties. The present correlation measurement is not affected by systematics in the measurement of the depth of shower maximum or the signal in the water Cherenkov stations. The analysis relies on general characteristics of air showers and is thus robust also with respect to uncertainties in hadronic event generators. The observed correlation in the energy range around the 'ankle' at lg⁡(E/eV)=18.5–19.0 differs significantly from expectations for pure primary cosmic-ray compositions. A light composition made up of proton and helium only is equally inconsistent with observations. The data are explained well by a mixed composition including nuclei with mass A>4. Scenarios such as the proton dip model, with almost pure compositions, are thus disfavored as the sole explanation of the ultrahigh-energy cosmic-ray flux at Earth.
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