Oh-My-God particle

On 15 October 1991 the Fly's Eye detector in the Utah desert recorded a single particle carrying about 3.2×10²⁰ electronvolts. That is tens of millions of times the energy of a proton in the Large Hadron Collider, and about the energy of a pitched baseball squeezed into one subatomic particle.

A cosmic ray detected in 2021 came close (about 2.4×10²⁰ eV), but the 1991 event remains the strongest recorded. Where it came from is still unknown.

Context

The detector, known as Fly's Eye, was run by the University of Utah at the Dugway Proving Ground. It detected the faint glow of nitrogen in the air that is excited when a cosmic ray hits the atmosphere and triggers a shower of secondary particles.

Energy of 3.2 times 10 to the 20 electronvolts is about 51 joules, comparable to a baseball thrown at a moderate speed. At this energy, particles should lose energy through interactions with background radiation over long distances, so its source must be relatively near by cosmic standards, yet no obvious source has been identified.

In 2023, the Telescope Array collaboration announced a particle of about 2.4 times 10 to the 20 electronvolts, detected in 2021 and nicknamed Amaterasu. It is the second highest on record. The mystery of where such rare particles come from remains unresolved.