Observation of the GZK Cutoff Using the HiRes Detector

The High Resolution Fly's Eye (HiRes) experiment has observed the GZK cutoff. HiRes observes two features in the ultra-high energy cosmic ray (UHECR) flux spectrum: the Ankle at an energy of 4×1018 eV and a high energy suppression at 6×1019 eV. The later feature is at exactly the right energy for the GZK cutoff according to the E1/2 criterion. HiRes cannot claim to observe a third feature at lower energies, the Second Knee. The HiRes monocular spectra are presented, along with data demonstrating our control and understanding of systematic uncertainties affecting the energy and flux measurements.