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# Greisen-Zatsepin-Kuzmin limit

The Greisen-Zatsepin-Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic rays from distant sources.

## Computation of the GZK-limit

This limit was computed in 1966 by Kenneth Greisen and Vadim Kuzmin and Georgiy Zatsepin independently; based on interactions predicted between the cosmic ray and the photons of the cosmic microwave background radiation. They predicted that cosmic rays with energies over the threshold energy of 6×1019 eV would interact with cosmic microwave background photons to produce pions. This would continue until their energy fell below the pion production threshold.

$gamma+prightarrowDelta^+rightarrow p + pi^0$

or

$gamma+prightarrowDelta^+rightarrow n + pi^+$

Because of the mean path associated with the interaction, extragalactic cosmic rays with distances more than 50 Mpc (163 Mly) from the Earth with energies greater than this threshold energy should never be observed on Earth, and there are no known sources within this distance that could produce them.

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 appeared 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.

• The observations could be due to an instrument error or an incorrect interpretation of the experiment.
• The cosmic rays could have local sources (although it is unclear what these sources could be).
• Heavier nuclei could possibly circumvent the GZK limit.

### Weakly interacting particles

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.

### Proposed theories for particles above the GZK-cutoff

A number of exotic theories have been advanced to explain the AGASA observations.

The most notable is the theory of doubly-special relativity. However, it is now established that standard doubly special relativity does not predict any suppression of the GZK cutoff, contrary to the pattern explored since 1997 by Luis Gonzalez-Mestres where an absolute local rest frame (the "vacuum rest frame") exists.

Other possible theories involve a relation with dark matter.

## Conflicting evidence for GZK-cutoff

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. The Auger Observatory has confirmed this result: 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.

### Extreme Universe Space Observatory (EUSO)

EUSO which is scheduled to fly on the International Space Station 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.

### GLAST to resolve inconsistencies

Launched in June 2008, Gamma-ray Large Area Space Telescope (GLAST) will also provide data that will help resolve these inconsistencies.

• With GLAST, one has the possibility of detecting gamma rays from the freshly accelerated cosmic-ray nuclei at their acceleration site (the source of the UHECRs).
• UHECR protons accelerated in astrophysical objects produce secondary electromagnetic cascades during propagation in the cosmic microwave and infrared backgrounds, of which the GZK-process of pion production is one of the contributors. Such cascades can contribute between ≃1% and ≃50% of the GeV-TeV diffuse photon flux measured by the EGRET experiment. The GLAST satellite may discover this flux.

## Possible sources of UHECRs

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 array of 1600 water tank detectors covering of land in Argentina. The observatory began operation in 2004. The results are reported in the journal Science.

## Pierre Auger Observatory results on UHECRs above GZK-limit

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, and that a delayed suppression of the GZK cutoff cannot yet be excluded.