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In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray particle with a kinetic energy greater than eV, far beyond both its rest mass and energies typical of other cosmic ray particles. An extreme-energy cosmic ray (EECR) is an UHECR with energy exceeding (about 8 joule), the so-called Greisen–Zatsepin–Kuzmin limit (GZK limit). This limit should be the maximum energy of cosmic ray particles that have traveled long distances (about 160 million light years), since higher-energy ray particles would have lost energy over that distance due to scattering from photons in the cosmic microwave background. 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. These particles are extremely rare; between 2004 and 2007, the initial runs of the Pierre Auger Observatory detected 27 events with estimated arrival energies above , i.e., about one such event every four weeks in the 3000 km2 area surveyed by the observatory.〔 〕 There is evidence that these highest-energy cosmic rays might be iron nuclei, rather than the protons that make up most cosmic rays. 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.〔 〕 In 2007, PAO tentatively associated EECR with extragalactic supermassive black holes at the center of nearby galaxies called active galactic nuclei (AGN).〔 〕 Extremely high energies might be explained also by the Centrifugal mechanism of acceleration 〔(Osmanov Z., Mahajan S., Machabeli G. & Chkheidze N. Monthly Notices of the Royal Astronomical Society, Volume 445, Issue 4, p.4155-4160 )〕 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.〔 A more speculative suggestion by Grib and Pavlov (2007, 2008) envisages the decay of superheavy dark matter by means of the Penrose process. ==Observational history== (詳細はJohn D Linsley and Livio Scarsi at the Volcano Ranch experiment in New Mexico in 1962.〔 〕〔 〕 Cosmic ray particles with even higher energies have since been observed. Among them was the Oh-My-God particle observed on the evening of 15 October 1991 over Dugway Proving Ground, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately (50 J)〔 〕—in other words, an atomic nucleus with kinetic energy equal to that of a baseball () traveling at about . The energy of this particle is some 40 million times that of the highest energy protons that have been produced in any terrestrial particle accelerator. However, only a small fraction of this energy would be available for an interaction with a proton or neutron on Earth, with most of the energy remaining in the form of kinetic energy of the products of the interaction. The effective energy available for such a collision is the square root of double the product of the particle's energy and the mass energy of the proton, which for this particle gives , roughly 50 times the collision energy of the Large Hadron Collider. 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. These very high energy cosmic ray particles are very rare; the energy of most cosmic ray particles is between 10 MeV and 10 GeV. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ultra-high-energy cosmic ray」の詳細全文を読む スポンサード リンク
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