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The Greisen-Zatsepin-Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic rays from distant sources.
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This extra limit was independently computed in 1966 by Kenneth Greisen[1], and Vadim Kuz'min and Georgiy Zatsepin[2], based on interactions between the cosmic ray and the photons of the cosmic microwave background radiation (CMB). They predicted that cosmic rays with energies over the threshold energy of 5×1019 eV would interact with cosmic microwave background photons γCMB to produce pions via the Δ resonance,
or
This process continues until the cosmic ray energy falls below the pion production threshold. Due to the mean path associated with this interaction, extragalactic cosmic rays traveling over distances larger than 50 Mpc (163 Mly) and with energies greater than this threshold should never be observed on Earth. This distance is also known as GZK horizon.
A number of observations have been made by the AGASA experiment that appeared to show cosmic rays from distant sources with energies above this limit (called ultra-high-energy cosmic rays, or UHECRs). The observed existence of these particles was the so-called GZK paradox or cosmic ray paradox.
These observations appear to contradict the predictions of special relativity and particle physics as they are presently understood. However, there are a number of possible explanations for these observations that may resolve this inconsistency.
Another suggestion involves ultra-high energy weakly interacting particles (for instance, neutrinos) which might be created at great distances and later react locally to give rise to the particles observed.
A number of exotic theories have been advanced to explain the AGASA observations, the most notable being the theory of doubly-special relativity. However, it is now established that standard doubly special relativity does not predict any GZK suppression (or GZK cutoff). Other possible theories involve a relation with dark matter, decays of exotic super-heavy particles beyond those known in the Standard Model.
Another possibility is that the ultra-energetic cosmic rays are composed of neutrons, which would be much less affected by the cosmic microwave background radiation. Such ultra-relativistic neutrons would be produced by collisions between original ultra-relativistic charged particles and helium (or heavier) nuclei. These neutrons would pass through the cosmic microwave background radiation without much hindrance. The spontaneous decay of the neutron is no hindrance here, since the time dilation occurring for such a neutron would make their passage from one end of the Universe to the other take only a few seconds in the neutron's rest frame.
In July 2007, during the 30th International Cosmic Ray Conference in Mérida, Yucatán, México, the High Resolution Fly's Eye Experiment (HiRes) and the Auger International Collaboration presented their results on ultra-high-energy cosmic rays. HiRes has observed a suppression in the UHECR spectrum at just the right energy, observing only 13 events with an energy above the threshold, while expecting 43 with no suppression. This result has been published in the Physical Review Letters in 2008 and as such is the first observation of the GZK Suppression.[3] The Auger Observatory has confirmed this result[4]: instead of the 30 events necessary to confirm the AGASA results, Auger saw only two, which are believed to be heavy nuclei events. According to Alan Watson, spokesperson for the Auger Collaboration, AGASA results have been shown to be incorrect, possibly due to the systematical shift in energy assignment.
EUSO which is scheduled to fly on the International Space Station (ISS) in 2009, will use the atmospheric-fluorescence technique to monitor a huge area and boost the statistics of UHECRs considerably. EUSO will make a deep survey of UHECR-induced extensive air showers (EASs) from space, extending the measured energy spectrum well beyond the GZK-cutoff. It will search for the origin of UHECRs, determine the nature of the origin of UHECRs, make an all-sky survey of the arrival direction of UHECRs, and seek to open the astronomical window on the extreme-energy universe with neutrinos. The fate of the EUSO Observatory is still unclear since NASA is considering early retirement of the ISS.
Launched in June 2008, the Fermi Gamma-ray Space Telescope (formerly GLAST) will also provide data that will help resolve these inconsistencies.
In November 2007, researchers at the Pierre Auger Observatory announced that they had evidence that UHECRs appear to come from the active galactic nuclei (AGNs) of energetic galaxies powered by matter swirling onto a supermassive black hole. The cosmic rays were detected and traced back to the AGNs using the Véron-Cetty-Véron catalog. These results are reported in the journal Science.[7] Nevertheless, the strength of the correlation with AGNs from this particular catalog for the Auger data recorded after 2007 has been slowly diminishing.
According to the analysis made by the AUGER collaboration the existence of the GZK cutoff seems to be confirmed, but it has been pointed out that the consequences of this result for models of Lorentz symmetry violation may depend crucially on the composition of the UHECR spectrum,[8] and that a delayed suppression of the GZK cutoff cannot yet be excluded.
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