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First Observation of the Greisen-Zatsepin-Kuzmin Suppression

HiRes Collaboration
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Abstract

The High Resolution Fly's Eye (HiRes) experiment has observed the Greisen-Zatsepin-Kuzmin suppression (called the GZK cutoff) with a statistical significance of five standard deviations. HiRes' measurement of the flux of ultrahigh energy (UHE) cosmic rays shows a sharp suppression at an energy of $6 \times 10^{19}$ eV, consistent with the expected cutoff energy. We observe the ``ankle'' of the cosmic-ray energy spectrum as well, at an energy of $4 \times 10^{18}$ eV. We describe the experiment, data collection, analysis, and estimate the systematic uncertainties. The results are presented and the calculation of the statistical significance of our observation is described.

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HiRes Collaboration. 2008-02-15. First Observation of the Greisen-Zatsepin-Kuzmin Suppression. https://doi.org/10.1103/physrevlett.100.101101

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