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ⓘ Greisen–Zatsepin–Kuzmin limit. The Greisen–Zatsepin–Kuzmin limit is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies ..




                                     

ⓘ Greisen–Zatsepin–Kuzmin limit

The Greisen–Zatsepin–Kuzmin limit is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy. The limit is 5 × 10 19 eV, or about 8 joules. The limit is set by slowing interactions of the protons with the microwave background radiation over long distances. The limit is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected. For example, one extreme-energy cosmic ray, the Oh-My-God Particle, which has been found to possess a record-breaking 3.12 × 10 20 eV of energy.

The GZK limit is derived under the assumption that ultra-high energy cosmic rays are protons. Measurements by the largest cosmic-ray observatory, the Pierre Auger Observatory, suggest that most ultra-high energy cosmic rays are heavier elements. In this case, the argument behind the GZK limit does not apply in the originally simple form, and there is no fundamental contradiction in observing cosmic rays with energies that violate the limit.

In the past, the apparent violation of the GZK limit has inspired cosmologists and theoretical physicists to suggest other ways that circumvent the limit. These theories propose that ultra-high energy cosmic rays are produced nearby our galaxy or that Lorentz covariance is violated in such a way that protons do not lose energy on their way to our galaxy.

                                     

1. Computation

The limit was independently computed in 1966 by Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin based on interactions between cosmic rays and the photons of the cosmic microwave background radiation CMB. They predicted that cosmic rays with energies over the threshold energy of 5 × 10 19 eV would interact with cosmic microwave background photons γ C M B {\displaystyle \gamma _{\rm {CMB}}}, relatively blueshifted by the speed of the cosmic rays, to produce pions through the Δ {\displaystyle \Delta } resonance,

γ CMB + p → Δ + → p + π 0, {\displaystyle \gamma _{\text{CMB}}+p\to \Delta ^{+}\to p+\pi ^{0},}

or

γ CMB + p → Δ + → n + π +. {\displaystyle \gamma _{\text{CMB}}+p\to \Delta ^{+}\to n+\pi ^{+}.}

Pions produced in this manner proceed to decay in the standard pion channels - ultimately to photons for neutral pions, and photons, positrons, and various neutrinos for positive pions. Neutrons decay also to similar products, so that ultimately the energy of any cosmic ray proton is drained off by production of high-energy photons plus in some cases high-energy electron–positron pairs and neutrino pairs.

The pion production process begins at a higher energy than ordinary electron-positron pair production lepton production from protons impacting the CMB, which starts at cosmic-ray proton energies of only about 10 17 eV. However, pion production events drain 20% of the energy of a cosmic-ray proton as compared with only 0.1% of its energy for electron–positron pair production. This factor of 200 is from two sources: the pion has only about ~130 times the mass of the leptons, but the extra energy appears as different kinetic energies of the pion or leptons and results in relatively more kinetic energy transferred to a heavier product pion, in order to conserve momentum. The much larger total energy losses from pion production result in the pion production process becoming the limiting one to high-energy cosmic-ray travel, rather than the lower-energy light-lepton production process.

The pion production 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.

                                     

2. Cosmic-ray paradox

A number of observations have been made by the largest cosmic-ray experiments Akeno Giant Air Shower Array, High Resolution Flys Eye Cosmic Ray Detector, the Pierre Auger Observatory and Telescope Array Project that appeared to show cosmic rays with energies above this limit called extreme-energy cosmic rays, or EECRs. The observation 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.

  • The observations could be due to an instrument error or an incorrect interpretation of the experiment, especially wrong energy assignment.
  • The cosmic rays could have local sources well within the GZK horizon although it is unclear what these sources could be.
                                     

2.1. Cosmic-ray paradox 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. In the proposed Z-burst model, an ultra-high-energy cosmic neutrino collides with a relic anti-neutrino in our galaxy and annihilates to hadrons. This process proceeds through a virtual Z-boson:

ν + ν ¯ → Z → hadrons. {\displaystyle \nu +{\bar {\nu }}\to Z\to {\text{hadrons}}.}

The cross-section for this process becomes large if the center-of-mass energy of the neutrino antineutrino pair is equal to the Z-boson mass such a peak in the cross-section is called "resonance". Assuming that the relic anti-neutrino is at rest, the energy of the incident cosmic neutrino has to be

E = m Z 2 m ν = 4.2 × 10 21 eV m ν eV, {\displaystyle E={\frac {m_{Z}^{2}}{2m_{\nu }}}=4.2\times 10^{21}\left{\frac {\text{eV}}{m_{\nu }}}\right~{\text{eV}},}

where m Z {\displaystyle m_{Z}} is the mass of the Z-boson, and m ν {\displaystyle m_{\nu }} the mass of the neutrino.



                                     

2.2. Cosmic-ray paradox Other theories

A number of exotic theories have been advanced to explain the AGASA observations, including doubly special relativity. However, it is now established that standard doubly special relativity does not predict any GZK suppression or GZK cutoff, contrary to models of Lorentz symmetry violation involving an absolute rest frame. Other possible theories involve a relation with dark matter, decays of exotic super-heavy particles beyond those known in the Standard Model.

                                     

3. Controversy about cosmic rays above the GZK limit

A suppression of the cosmic-ray flux that can be explained with the GZK limit has been confirmed by the latest generation of cosmic-ray observatories. A former claim by the AGASA experiment that there is no suppression was overruled. It remains controversial whether the suppression is due to the GZK effect. The GZK limit only applies if ultra-high-energy cosmic rays are mostly protons.

In July 2007, during the 30th International Cosmic Ray Conference in Merida, Yucatan, Mexico, the High Resolution Flys Eye Experiment HiRes and the Pierre Auger Observatory Auger presented their results on ultra-high-energy cosmic rays. HiRes 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 was interpreted as the first observation of the GZK limit. Auger confirmed the flux suppression, but did not claim it to be the GZK limit: instead of the 30 events necessary to confirm the AGASA results, Auger saw only two, which are believed to be heavy-nuclei events. The flux suppression was previously brought into question when the AGASA experiment found no suppression in their spectrum. According to Alan Watson, spokesperson for the Auger Collaboration, AGASA results have been shown to be incorrect, possibly due to the systematic shift in energy assignment.

In 2010 and the following years, both the Pierre Auger Observatory and HiRes confirmed again a flux suppression, in case of the Pierre Auger Observatory the effect is statistically significant at the level of 20 standard deviations.

After the flux suppression was established, a heated debate ensued whether cosmic rays that violate the GZK limit are protons. The Pierre Auger Observatory, the worlds largest observatory, found with high statistical significance that ultra-high-energy cosmic rays are not purely protons, but a mixture of elements, which is getting heavier with increasing energy. The Telescope Array Project, a joint effort from members of the HiRes and AGASA collaborations, agrees with the former HiRes result that these cosmic rays look like protons. The claim is based on data with lower statistical significance, however. The area covered by Telescope Array is about one third of the area covered by the Pierre Auger Observatory, and the latter has been running for a longer time.

The controversy was partially resolved in 2017, when a joint working group formed by members of both experiments presented a report at the 35th International Cosmic Ray Conference. According to the report, the raw experimental results are not in contradiction with each other. The different interpretations are mainly based on the use of different theoretical models Telescope Array uses an outdated model for its interpretation and the fact that Telescope Array has not collected enough events yet to distinguish the pure-proton hypothesis from the mixed-nuclei hypothesis.

                                     

3.1. Controversy about cosmic rays above the GZK limit Extreme Universe Space Observatory on Japanese Experiment Module JEM-EUSO

EUSO, which was scheduled to fly on the International Space Station ISS in 2009, was designed to use the atmospheric-fluorescence technique to monitor a huge area and boost the statistics of UHECRs considerably. EUSO is to make a deep survey of UHECR-induced extensive air showers EASs from space, extending the measured energy spectrum well beyond the GZK cutoff. It is to 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.

                                     

3.2. Controversy about cosmic rays above the GZK limit The Fermi Gamma-ray Space Telescope to resolve inconsistencies

Launched in June 2008, the Fermi Gamma-ray Space Telescope formerly GLAST will also provide data that will help resolve these inconsistencies.

  • UHECR protons accelerated see also Centrifugal mechanism of acceleration 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 about 1% and 50% of the GeV–TeV diffuse photon flux measured by the EGRET experiment. The Fermi Gamma-ray Space Telescope may discover this flux.
  • With the Fermi Gamma-ray Space Telescope, one has the possibility of detecting gamma rays from the freshly accelerated cosmic-ray nuclei at their acceleration site the source of the UHECRs.
                                     

4. 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 Veron-Cetty-Veron catalog. These results are reported in the journal Science. Nevertheless, the strength of the correlation with AGNs from this particular catalog for the Auger data recorded after 2007 has been slowly diminishing.

                                     
  • Gregory Wright astrophysicist Gregory Laflamme instability Greisen Zatsepin Kuzmin limit Grenville Turner Gretar Tryggvason Grey noise Gribov ambiguity
  • upper limit on the energies of cosmic rays surviving collisions with the cosmic microwave background, through the Greisen Zatsepin Kuzmin limit In the
  • 2006. The HiRes detector made the first observation of the Greisen Zatsepin Kuzmin limit which indicates the highest energy cosmic rays that interact
  • rays would likely come from outside our galaxy. According to the Greisen Zatsepin Kuzmin GZK cutoff, inter - galactic cosmic rays above 1020eV should be
  • violations. Other candidates for astronomical observations are the Greisen Zatsepin Kuzmin limit and Airy disks. The latter is investigated to find possible
  • of 1018 eV, one million times more powerful  the so - called Greisen Zatsepin Kuzmin limit and other cosmic events may be more powerful yet. Against
  • the galaxy, and their existence is not consistent with the Greisen - Zatsepin - Kuzmin limit On 4 October 1957, the Soviet Union launched the first artificial
  • Glashow Weinberg Salem theory of the electroweak force. GZK The Greisen Zatsepin Kuzmin limit on the energy of cosmic background radiation from distant sources
  • Extreme - energy cosmic rays have been observed that seem to violate the Greisen Zatsepin Kuzmin limit which is a consequence of special relativity. Paradox of youth:
  • Bibcode: 1995dmcc.conf..645G. Luis Gonzalez - Mestres 1997 - 05 - 26 Absence of Greisen - Zatsepin - Kuzmin Cutoff and Stability of Unstable Particles at Very High Energy

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