Greisen-Zatsepin-Kuzmin limit
Encyclopedia
The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic ray
s (high energy charged particles from space) coming from "distant" sources. The limit is 5×1019 eV
, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances (~163 million light-years). The limit, interestingly, is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected. For example, one ultra-high-energy cosmic ray
has been detected which appeared to possess a record 50 joules of energy (about the same as a 60 mph baseball).
Cosmologists and theoretical physicists have regarded such observations as key in the search for explorations of physics in the energy realms which would require new theories of quantum gravity
and other theories which predict events at the Planck scale
. This is because protons at these extreme energies (3 million TeV) are much closer to the Planck energy (about 2 billion joules) than any particles that can be made by current particle accelerators (20 TeV, or 3 millionths of a joule). They are thus suitable as a probe into realms where the theory of special relativity
breaks down. Physicist Lee Smolin has written that if such cosmic rays which violate the GZK limit can be confirmed, and other possible explanations discounted, it "would be the most momentous discovery of the last hundred years—the first breakdown of the basic theories comprising the twentieth century's scientific revolution."
, based on interactions between cosmic ray
s 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
to produce pion
s via the
resonance,
or
The h superscript is chosen to conserve charge, so that neutral pions are produced if a proton enters and exits the reaction, while a positive pion must be produced if a proton enters and a neutron leaves. 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 1017eV. 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 becomming 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.
s, 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.
s) 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 cosmic neutrino collides with a relic anti-neutrino in our galaxy and annihilates to hadrons. This process proceeds via a (virtual) Z-boson:
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:
where
is the mass of the Z-boson and 
the mass of the neutrino.
.
(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, possibly due to the systematical shift in energy assignment.
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.
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
. Nevertheless, the strength of the correlation with AGNs from this particular catalog for the Auger data recorded after 2007 has been slowly diminishing.
Furthermore, it has been pointed out by Luis Gonzalez-Mestres 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.
In 2010 final results of The High Resolution Fly's Eye (HiRes)
experiment reconfirmed earlier results of the GZK cutoff from the HiRes experiment. The results were previously brought into question when the AGASA
experiment hinted at suppression of the GZK cutoff in their spectrum. The AUGER collaboration results agree with some parts of the HiRes final results on the GZK cutoff, but some discrepancies still remain.
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s (high energy charged particles from space) coming from "distant" sources. The limit is 5×1019 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
, or about 8 joules. The limit is set by slowing-interactions of cosmic ray protons with the microwave background radiation over long distances (~163 million light-years). The limit, interestingly, is at the same order of magnitude as the upper limit for energy at which cosmic rays have experimentally been detected. For example, one ultra-high-energy cosmic ray
Ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray or extreme-energy cosmic ray is a cosmic ray with an extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays....
has been detected which appeared to possess a record 50 joules of energy (about the same as a 60 mph baseball).
Cosmologists and theoretical physicists have regarded such observations as key in the search for explorations of physics in the energy realms which would require new theories of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
and other theories which predict events at the Planck scale
Planck scale
In particle physics and physical cosmology, the Planck scale is an energy scale around 1.22 × 1019 GeV at which quantum effects of gravity become strong...
. This is because protons at these extreme energies (3 million TeV) are much closer to the Planck energy (about 2 billion joules) than any particles that can be made by current particle accelerators (20 TeV, or 3 millionths of a joule). They are thus suitable as a probe into realms where the theory of special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
breaks down. Physicist Lee Smolin has written that if such cosmic rays which violate the GZK limit can be confirmed, and other possible explanations discounted, it "would be the most momentous discovery of the last hundred years—the first breakdown of the basic theories comprising the twentieth century's scientific revolution."
Computation of the GZK-limit
The limit was independently computed in 1966 by Kenneth Greisen, Vadim Kuzmin, and Georgiy ZatsepinGeorgiy Zatsepin
Georgiy Timofeyevich Zatsepin was a Soviet/Russian astrophysicist known for his works in cosmic rays physics and neutrino astrophysics. He was born in Moscow....
, based on interactions between cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s and the photons of the cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
(CMB). They predicted that cosmic rays with energies over the threshold energy of 5×1019 eV would interact with cosmic microwave background photons
to produce pionPion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s via the
Delta baryon
The Delta baryons are a family of subatomic hadron particles which have the symbols , , , and and electric charges +2, +1, 0 and -1 elementary charge respectively...
resonance,
or
The h superscript is chosen to conserve charge, so that neutral pions are produced if a proton enters and exits the reaction, while a positive pion must be produced if a proton enters and a neutron leaves. 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 1017eV. 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 becomming 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
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
(163 Mly
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
) and with energies greater than this threshold should never be observed on Earth. This distance is also known as GZK horizon.
Cosmic ray paradox
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 rayUltra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray or extreme-energy cosmic ray is a cosmic ray with an extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays....
s, 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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
and particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
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).
- Heavier nuclei could possibly circumvent the GZK limit.
Weakly interacting particles
Another suggestion involves ultra-high energy weakly interacting particles (for instance, neutrinoNeutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s) 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 cosmic neutrino collides with a relic anti-neutrino in our galaxy and annihilates to hadrons. This process proceeds via a (virtual) Z-boson:
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:
where
is the mass of the Z-boson and the mass of the neutrino.Proposed theories for particles above the GZK-cutoff
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 ModelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
.
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 ExperimentHigh Resolution Fly's Eye Cosmic Ray Detector
The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the western Utah desert from May 1997 until April 2006. HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch...
(HiRes) and the Auger International Collaboration
Pierre Auger Observatory
The Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: single sub-atomic particles with energies beyond 1020 eV...
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
Physical Review Letters
Physical Review Letters , established in 1958, is a peer reviewed, scientific journal that is published 52 times per year by the American Physical Society...
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, possibly due to the systematical shift in energy assignment.
Extreme Universe Space Observatory (EUSO)
EUSOExtreme Universe Space Observatory
The Extreme Universe Space Observatory is the first Space mission concept devoted to the investigation of cosmic rays and neutrinos of extreme energy...
which was scheduled to fly on the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
(ISS) in 2009, was designed to use the atmospheric-fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
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.
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.- 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).
- 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 EGRETEgretAn egret is any of several herons, most of which are white or buff, and several of which develop fine plumes during the breeding season. Many egrets are members of the genera Egretta or Ardea which contain other species named as herons rather than egrets...
experiment. The Fermi Gamma-ray Space Telescope may discover this flux.
Possible sources of UHECRs
In November 2007, researchers at the Pierre Auger ObservatoryPierre Auger Observatory
The Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: single sub-atomic particles with energies beyond 1020 eV...
announced that they had evidence that UHECRs appear to come from the active galactic nuclei
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
(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
Philippe Véron
Philippe Véron is a French astronomer. He works at Observatoire de Haute Provence, whom he was director. He studies variability and statistics of quasars, as well as elliptical galaxies. He is married with French astronomer Marie-Paule Véron-Cetty....
catalog. These results are reported in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
. Nevertheless, the strength of the correlation with AGNs from this particular catalog for the Auger data recorded after 2007 has been slowly diminishing.
Pierre Auger Observatory results on UHECRs above GZK-limit
According to the analysis made by the AUGER collaboration, the existence of the GZK cutoff may have been confirmed, but important uncertainties remain in the interpretation of the experimental results and further work is required.Furthermore, it has been pointed out by Luis Gonzalez-Mestres 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.
In 2010 final results of The High Resolution Fly's Eye (HiRes)
High Resolution Fly's Eye Cosmic Ray Detector
The High Resolution Fly's Eye or HiRes detector was an ultra-high-energy cosmic ray observatory that operated in the western Utah desert from May 1997 until April 2006. HiRes utilized the atmospheric fluorescence technique that was pioneered by the Utah group first in tests at the Volcano Ranch...
experiment reconfirmed earlier results of the GZK cutoff from the HiRes experiment. The results were previously brought into question when the AGASA
AGASA
The Akeno Giant Air Shower Array is a very large surface array designed to study the origin of ultra-high-energy cosmic rays. It covers an area of 100 km2 and consists of 111 surface detectors and 27 muon detectors...
experiment hinted at suppression of the GZK cutoff in their spectrum. The AUGER collaboration results agree with some parts of the HiRes final results on the GZK cutoff, but some discrepancies still remain.
External links
- Rutgers University experimental high energy physics HIRES research page
- Pierre Auger Observatory page
- Cosmic-ray.org
- "Could the end be in sight for ultrahigh-energy cosmic rays?", Subir Sarkar, PhysicsWeb, 2002
- History of Cosmic Ray Research
- Vacuum Structure, Lorentz Symmetry and Superluminal Particles, by L. Gonzalez-Mestres, and other papers by the same author.