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In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray particles.

An extreme-energy cosmic ray (EECR) is an UHECR with energy exceeding 5×1019 eV (about 8 joule), the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). This limit should be the maximum energy of cosmic ray protons that have traveled long distances (about 160 million light years), since higher-energy protons would have lost energy over that distance due to scattering from photons in the cosmic microwave background (CMB). It follows that EECR could not be survivors from the early universe, but are cosmologically "young", emitted somewhere in the Local Supercluster by some unknown physical process. If an EECR is not a proton, but a nucleus with A nucleons, then the GZK limit applies to its nucleons, which carry only a fraction 1/A of the total energy of the nucleus. For an iron nucleus, the corresponding limit would be 2.8×1021 eV. However, nuclear physics processes lead to limits for iron nuclei similar to that of protons. Other abundant nuclei have even much lower limits.

These particles are extremely rare; between 2004 and 2007, the initial runs of the Pierre Auger Observatory (PAO) detected 27 events with estimated arrival energies above 5.7×1019 eV, that is, about one such event every four weeks in the 3000 km2 area surveyed by the observatory.[1]

There is evidence that these highest-energy cosmic rays might be iron nuclei, rather than the protons that make up most cosmic rays.[2]

The postulated (hypothetical) sources of EECR are known as Zevatrons, named in analogy to Lawrence Berkeley National Laboratory's Bevatron and Fermilab's Tevatron, and therefore capable of accelerating particles to 1 ZeV (1021 eV, zetta-electronvolt). In 2004 there was a consideration of the possibility of galactic jets acting as Zevatrons, due to diffusive acceleration of particles caused by shock waves inside the jets. In particular, models suggested that shock waves from the nearby M87 galactic jet could accelerate an iron nucleus to ZeV ranges.[3] In 2007, the Pierre Auger Observatory observed a correlation of EECR with extragalactic supermassive black holes at the center of nearby galaxies called active galactic nuclei (AGN).[4] However, the strength of the correlation became weaker with continuing observations. Extremely high energies might be explained also by the centrifugal mechanism of acceleration [5] in the magnetospheres of AGN, although newer results indicate that fewer than 40% of these cosmic rays seemed to be coming from the AGN, a much weaker correlation than previously reported.[2] A more speculative suggestion by Grib and Pavlov (2007, 2008) envisages the decay of superheavy dark matter by means of the Penrose process.