Ultra-high-energy cosmic ray

In high-energy physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray (subatomic particle) which appears to have extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays. These particles are significant because they have energy comparable to (and sometimes exceeding) the Greisen-Zatsepin-Kuzmin limit.

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The Oh-My-God particle (a play on the nickname "God particle" for the Higgs boson) is the nickname given to a particle observed on the evening of October 15, 1991, over Dugway Proving Grounds, Utah, estimated to have an energy of approximately 3 × 10²⁰ electronvolts, equivalent to about 50 joules — in other words, it was a subatomic particle with macroscopic kinetic energy equal to that of a baseball (140 g) which is moving at about 27 m/s (60 mph). These very high energy cosmic rays are however very rare and most cosmic rays possess an energy between 10⁷ eV and 10¹⁰ eV.

It was most likely a proton travelling with velocity almost equal to the speed of light (if it was a proton, its speed would have been approximately (1 − 5 × 10⁻²⁴) c; after traveling one year the particle would be only 46 nanometres behind a photon that left at the same time^[1]) and its observation was a shock to astrophysicists.

Since the first observation, by the University of Utah's Fly's Eye Cosmic Ray Detector, at least fifteen similar events have been recorded, confirming the phenomenon. The source of such high energy particles was a mystery for many years, but results later correlated ultra high energy cosmic ray origins with extragalactic super-massive black holes at the center of nearby galaxies called active galactic nuclei.^[2] Interactions with blue-shifted cosmic microwave background radiation limit the distance that these particles can travel before losing energy (the Greisen-Zatsepin-Kuzmin limit).

Because of its energy, the Oh-My-God particle would have experienced very little influence from cosmic electromagnetic and gravitational fields, and so its trajectory should be easily calculable. However, nothing of note was found in the estimated direction of its origin.

In November 2007, the Pierre Auger Observatory announced that they had found a correlation between the highest energy events and nearby active galactic nuclei and that the rapid decrease in the number of events at highest energy is consistent with the GZK process. This confirms the GZK cutoff even further, making such particles as the Oh-My-God particle even more mysterious.

See also

• MARIACHI
• GRAPES-3

References