Observation of the GZK Cutoff by the HiRes Experiment

The High Resolution Fly's Eye (HiRes) experiment has observed the GZK cutoff. HiRes' measurement of the flux of cosmic rays shows a sharp suppression at an energy of $6 \times 10^{19}$ eV, exactly the expected cutoff energy. We observe the ``Ankle'' of the cosmic ray 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 a $\sim5$ standard deviation observation of the GZK cutoff is described.