In October of 1991, The FE1 detector observed an air shower with an energy of 3.2x1020 eV. This corresponds to ~50 joules or ~12 calories, or roughly the kinetic energy of a well-pitched baseball. As of the year 2012, this remains the highest energy particle ever recorded from any source. A display of the event is shown below, where the x- and z-direction cosines of the hit pixels are circled.
The 3.2x1020 eV event was not seen by the FE2 detector. The absence of a corresponding trigger in FE2 is consist with the FE1 monocular reconstruction, which places the shower outside the field of view of FE2. The plot below shows the reconstrucetd shower profile. The observed Xmax of ~800 g/cm2 is perfectly consistent with a hadron-initiated air shower of the measured 3.2x1020 eV energy.
The most important aspect of this 3.2x1020 eV event is that it is ~5 times above the theoretical Greisen-Zaptsepin-K'uzmin (GZK) cut-off at ~6x1019 eV. Super-GZK events (those above 6x1019 eV) had previously been reported by a number of ground array experiments, including Volcano Ranch, Yakutsk, and Haverah Park. The Fly's Eye event is the first such event seen with the fluorescence technique and is siginificantly higher in energy than the other candidates. This discovery was seen to give strong evidence for a non-zero flux of cosmic rays above the GZK cut-off. Since the Fly's Eye event, the AGASA group has reported a number of super-GZK events. A discussion of the GZK cut-off is given in the Physics of HiRes section of this web-site.