Ultra-high-energy cosmic ray

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In high-energy physics, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) 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.

Contents 1 Observational history 2 Active galactic cores as source of the particles 3 Other possible sources of the particles 4 Relation with dark matter 4.1 Conversion of dark matter into ultra-high-energy particles 4.2 Dark matter particles as ultra-high-energy particles 5 Pierre Auger Observatory 6 Ultra-high-energy cosmic ray observatories 7 References 8 External links

[edit] Observational history

The first observation of a cosmic ray with an energy exceeding 10²⁰ electronvolts was made by John Linsley at the Volcanic Ranch experiment in New Mexico in 1962.^[1][2]

Cosmic rays with even higher energies have since been observed, among them the Oh-My-God particle (a play on the nickname "God particle" for the Higgs boson), observed on the evening of October 15, 1991, over Dugway Proving Grounds, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately 3 × 10²⁰ electronvolts (50 joules)— in other words, a subatomic particle with macroscopic kinetic energy equal to that of a baseball (140 g) thrown at 60 mph.

It was most likely a proton with a velocity almost equal to the speed of light. In a race, such a proton, traveling at [1 − (5×10⁻²⁴)] times c, would fall only 46 nanometers behind a photon after one year.^[3]

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 rays are however very rare and most cosmic rays possess an energy between 10⁷ eV and 10¹⁰ eV.

[edit] Active galactic cores as source of the particles

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.^[4] 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 27 highest energy events thus far detected, and nearby active galactic nuclei [AGN] and that the rapid decrease in the number of events at highest energy is consistent with the GZK process. This confirming the GZK cutoff even further.

Additional data collection is expected to obtain even stronger verification of the AGN source for these highest energy particles, which are believed to be protons accelerated to those energies by magnetic fields associated with the rapidly growing black holes at the AGN centers. According to a recent study ^[5], short-duration AGN flares resulting from the tidal disruption of a star or from a disk instability can be the main source of the observed flux of super GZK cosmic rays.

[edit] Other possible sources of the particles

Other possible sources of the UHECR are^[6]:

• radio lobes of powerful radio galaxies,
• intergalactic shocks created during the epoch of galaxy formation,
• hypernovae,
• gamma-ray bursts,
• decay products of super-massive particles from topological defects, left over from phase transitions in the early universe.

[edit] Relation with dark matter

Main article: dark matter

[edit] Conversion of dark matter into ultra-high-energy particles

It is hypothesized that active galactic nuclei are capable of converting dark matter into high energy protons. Yurii Pavlov at The Herzen University in St Petersburg and Grib, hypothesize that dark matter particles are about 15 times heavier than protons, and that they can decay into pairs of particles of a type that interacts.

Near an active galactic nucleus, one of these particles can fall into the black hole, while the other escapes, as described by the Penrose process. Some of the particles that escape will collide with incoming particles creating collisions of very high energy. It is in these collisions, according to Pavlov, that ordinary visible protons can form. These protons will have very high energies.

According to Pavlov evidence is present in the form of ultra high-energy cosmic rays.^[7]

[edit] Dark matter particles as ultra-high-energy particles

High energy cosmic rays traversing intergalactic space suffer the GZK cutoff above 100 × 10¹⁸ eV due to interactions with cosmic background radiation if the primary cosmic ray particles are protons or nuclei. The Pierre Auger Project, HiRes and Yakutsk Extensive Air Shower Array found the GZK cutoff, while Akeno-AGASA observed the events above the cutoff (11 events in the past 10 years). The result of the Akeno-AGASA experiment is smooth near the GZK cutoff energy. If one assumes that the Akeno-AGASA result is correct and consider its implication, a possible explanation for the AGASA data on GZK cutoff violation would be a shower caused by a dark matter particles. A dark matter particle is not constrained by the GZK cutoff, since it interacts weakly with cosmic background radiation. Recent measurements by the Pierre Auger Project have found a correlation between the direction of high energy cosmic rays and the location of AGN. ^[8]

[edit] Pierre Auger Observatory

Main article: Pierre Auger Observatory

Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra high energy cosmic rays (sub-atomic particles (protons or other nuclei) with energies beyond 10²⁰ electron-volts). These high energy particles have an estimated arrival rate of just 1 per square kilometer per century, therefore, in order to record a large number of these events, the Auger Observatory has created a detection area the size of Rhode Island in western Argentina's Mendoza Province.

A larger cosmic ray detector array is also planned for the northern hemisphere as part of the Pierre Auger complex.

The Pierre Auger Observatory, in addition to obtaining directional information from the cluster of water tanks used to observe the cosmic ray shower components, also has four telescopes trained on the night sky to observe fluorescence of the Nitrogen molecules as the shower particles traverse the sky, giving further directional information on the original cosmic ray.

[edit] Ultra-high-energy cosmic ray observatories

See also: Category:High energy particle telescopes

• MARIACHI - Mixed Apparatus for Radar Investigation of Cosmic-rays of High Ionization located on Long Island, USA.
• GRAPES-3 (Gamma Ray Astronomy PeV EnergieS 3rd establishment) is a project for cosmic ray study with air shower detector array and large area muon detectors at Ooty in southern India.
• LOPES (telescope) - LOFAR PrototypE Station is located in Karlsruhe, Germany is part of the LOFAR project.
• AGASA - Akeno Giant Air Shower Array in Japan
• High Resolution Fly's Eye Cosmic Ray Detector (HiRes)
• Yakutsk Extensive Air Shower Array
• Pierre Auger Observatory
• Extreme Universe Space Observatory

[edit] References

[edit] External links

• The Highest Energy Particle Ever Recorded The details of the event from the official site of the Fly's Eye detector.
• John Walker's lively analysis of the 1991 event, published in 1994
• Origin of energetic space particles pinpointed, by Mark Peplow for news@nature.com, published January 13, 2005.

• Ultra High Energy Cosmic Rays - (1908) Ritzian relativity in action?