Search Results (5 total)

The energy spectrum of cosmic rays above 2.5×10¹⁸ eV, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index γ of the particle flux, J∝E^-γ, at energies between 4×10¹⁸ eV and 4×10¹⁹ eV is 2.69±0.02(stat)±0.06(syst), steepening to 4.2±0.4(stat)±0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz’min.

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth’s crust. Tau leptons from ν_τ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ν_τ at EeV energies. Assuming an E_ν^-2 differential energy spectrum the limit set at 90% C.L. is E_ν²dN_ντ/dE_ν<1.3×10^-7  GeV cm^-2 s^-1 sr^-1 in the energy range 2×10¹⁷  eV<E_ν<2×10¹⁹  eV.

We have measured the cosmic ray spectrum above 10^17.2   eV using the two air-fluorescence detectors of the High Resolution Fly’s Eye observatory operating in monocular mode. We describe the detector, phototube, and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extragalactic sources.

The average mass composition of cosmic rays with primary energies between 10¹⁷ and 10¹⁸ eV has been studied using a hybrid detector consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum and the muon density as a function of energy. The results show that the composition is changing from a heavy to lighter mix as the energy increases.