Usuario:Jorgealda/GZK

El límite Greisen–Zatsepin–Kuzmin limit (límite GZK) es un límite superior teórico a la energía de los rayos cósmicos (partículas de alta energía procedentes del espacio) procedentes de fuentes distantes. El límite es 5×10¹⁹ eV, or aproximadamente 8 julios. El límite se debe a interacciones de los protones de los rayos cósmicos con el fondo cósmico de microondas a lo largo de grandes disntancias (~160 millones de años luz). El límite está en el mismo orden de magnitud que los rayos cósmicos más energéticos que se han observado.

Cálculo del límite GZK[editar]

El límite fue calculado de forma independiente en 1966 por Kenneth Greisen,^[1] Vadim Kuzmin, y Georgiy Zatsepin,^[2] empleando las intracciones de los rayos cósmicos y los fotones del fondo cósmico de microondas (CMB). Predijeron que los rayos cósmicos con energías por encima de la energía umbral de 5×10¹⁹ eV interaccionarían con los fotones del CMB $\gamma _{\rm {CMB}}$ , desplazados hacia el azul por la velocidad relativa de los rayos cósmicos, para producir piones mediante la resonancia del barión $\Delta$ ,

\gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow p+\pi ^{0},

o

\gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow n+\pi ^{+}.

Los piones producidos de este modo se desintegran en los canales habituales—los piones neutros en fotones, y los piones positivos en fotones, positrones y varios neutrinos. Los neutrones se desintegran en productos similares, de modo que la energía de cualquier protón de rayos cósmicos acaba en la producción de fotones de alta energía más (en algunos casos) pares de electrón-positrón y de neutrinos.

El proceso de producción de piones requiere de una energía mínima mayor que la producción de pares electrón-positrón (producción de leptones) debido a los protones que colisionan con el CMB, que comienza con rayos cósmicos de unos 10¹⁷eV. Sin embargo, la producción de piones emplea el 20% de la energía de un protón de rayos cósmicos frente al 0.1% que emplea la producción de leptones. El factor 200 se debe a dos motivos: el pión tiene unas 130 veces la masa de los leptones, y se requiere más energía cinética para mover un pión conservando el momento lineal. Las mayores pérdidas debidas a la producción de piones resultan en que este proceso sea el limitante para la propagación de rayos cósmicos energéticos.

La producción de piones continua hasta que la energía de los rayos cósmicos es menor que el umbral para la producción. Debido al camino libre medio asociado a la interacción, no se deberían observar en la Tierra rayos cósmicos extragalácticos que hayan recorrido más de 50 Mpc (163 millones de años luz) con energías superiores al umbral. Esta distancia se conoce como el horizonte GZK.

Observaciones[editar]

En 2004, el experimento Akeno Giant Air Shower Array (AGASA) publicó la existencia de rayos cósmicos procedentes de fuentes distantes que superarían el límite GZK.

En julio de 2007, durante la 30th International Cosmic Ray Conference celebrada en Mérida, Yucatán, México, el experimento High Resolution Fly's Eye Experiment (HiRes) y la colaboración Internacional Auger presentaron sus resultados sobre rayos cósmicos de ultra-alta energía. HiRes observó una supresión en el espectro de rayos cósmicos a la energía esperada, observando solo 13 eventos que superan el umbral, cuando se esperarían 43 eventos si no hubiera supresión. Este resultado fue publicado en Physical Review Letters en 2008, y es la primera observación de la supresión GZK.^[3] El Observatorio Auger confirmó este resultado:^[4] en lugar de los 30 eventos 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 on Japanese Experiment Module (JEM-EUSO)[editar]

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.

The Fermi Gamma-ray Space Telescope to resolve inconsistencies[editar]

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).^[5]
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 ≃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.^[6]

Paradoja de los rayos cósmicos[editar]

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).
Heavier nuclei could possibly circumvent the GZK limit.

Weakly interacting particles[editar]

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.^[7] This process proceeds via a (virtual) Z-boson:

$\nu +{\bar {\nu }}\rightarrow Z\rightarrow {\text{hadrons}}$

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:

$E={\frac {m_{Z}^{2}}{2m_{\nu }}}=4.2\times 10^{21}\left({\frac {\text{eV}}{m_{\nu }}}\right){\text{eV}}$

where $m_{Z}$ is the mass of the Z-boson and $m_{\nu }$ the mass of the neutrino.

Proposed theories for particles above the GZK-cutoff[editar]

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.^{[cita requerida]} Other possible theories involve a relation with dark matter, decays of exotic super-heavy particles beyond those known in the Standard Model.

Conflicting evidence for GZK-cutoff[editar]

Possible sources of UHECRs[editar]

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.^[8] Nevertheless, the strength of the correlation with AGNs from this particular catalog for the Auger data recorded after 2007 has been slowly diminishing.^[9]

Pierre Auger Observatory and HiRes results on UHECRs above GZK-limit[editar]

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.^[10]

In 2010 final results of The High Resolution Fly's Eye (HiRes) experiment reconfirmed earlier results of the GZK cutoff from the HiRes experiment.^[11] 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.

References[editar]

↑ Greisen, Kenneth (1966). «End to the Cosmic-Ray Spectrum?». Physical Review Letters 16 (17): 748–750. Bibcode:1966PhRvL..16..748G. doi:10.1103/PhysRevLett.16.748.
↑ Zatsepin, G. T.; Kuz'min, V. A. (1966). «Upper Limit of the Spectrum of Cosmic Rays». Journal of Experimental and Theoretical Physics Letters 4: 78–80. Bibcode:1966JETPL...4...78Z.
↑ Abbasi, R. U. (2008). «First Observation of the Greisen-Zatsepin-Kuzmin Suppression». Physical Review Letters 100 (10): 101101. Bibcode:2008PhRvL.100j1101A. PMID 18352170. arXiv:astro-ph/0703099. doi:10.1103/PhysRevLett.100.101101.
↑ Abraham, J. (2008). «Observation of the suppression of the flux of cosmic rays above 4×10¹⁹ eV». Physical Review Letters 101 (6): 061101-1-061101-7. Bibcode:2008PhRvL.101f1101A. arXiv:0806.4302. doi:10.1103/PhysRevLett.101.061101.
↑ Ormes, Jonathan F. (2000). «The origin of cosmic rays: What can the Fermi Gamma-ray Telescope say?». AIP Conference Proceedings 528: 445–448. arXiv:astro-ph/0003270. doi:10.1063/1.1324357.
↑ Kalashev, Oleg E.; Semikoz, Dmitry V.; Sigl, Guenter (2007). «Ultra-High Energy Cosmic Rays and the GeV-TeV Diffuse Gamma-Ray Flux». Physical Review D 79 (6): 063005. Bibcode:2009PhRvD..79f3005K. arXiv:0704.2463. doi:10.1103/PhysRevD.79.063005.
↑ Fargion, D.; Mele, B.; Salis, A. (June 1999). «Ultra–High‐Energy Neutrino Scattering onto Relic Light Neutrinos in the Galactic Halo as a Possible Source of the Highest Energy Extragalactic Cosmic Rays». The Astrophysical Journal 517 (2): 725-733. Bibcode:1999ApJ...517..725F. arXiv:astro-ph/9710029. doi:10.1086/307203.
↑ The Pierre Auger Collaboration (2007). «Correlation of the Highest-Energy Cosmic Rays with Nearby Extragalactic Objects». Science 318 (5852): 938–943. Bibcode:2007Sci...318..938T. PMID 17991855. arXiv:0711.2256. doi:10.1126/science.1151124.
↑ The Pierre Auger Collaboration (2010). «Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter». Astropart. Phys. 34 (5): 314–326. Bibcode:2010APh....34..314A. arXiv:1009.1855. doi:10.1016/j.astropartphys.2010.08.010.
↑ The Pierre Auger Collaboration (2010). «Measurement of the energy spectrum of cosmic rays above 10¹⁸ eV using the Pierre Auger Observatory». Phys. Lett. B 685 (4–5): 239–246. Bibcode:2010PhLB..685..239A. arXiv:1002.1975. doi:10.1016/j.physletb.2010.02.013.
↑ Sokolsky; for the HiRes Collaboration (2010). «Final Results from the High Resolution Fly's Eye (HiRes) Experiment». arXiv:1010.2690 [astro-ph.HE].

External links[editar]

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.

{{DEFAULTSORT:Greisen-Zatsepin-Kuzmin Limit}} [[Category:Cosmic rays]] [[Category:Physical paradoxes]] [[Category:Energy]] [[Category:Special relativity]] [[Category:Astroparticle physics]] [[Category:Unsolved problems in physics]] [[Category:Unsolved problems in astronomy]]

[Greisen1966-1] Greisen, Kenneth (1966). «End to the Cosmic-Ray Spectrum?». Physical Review Letters 16 (17): 748–750. Bibcode:1966PhRvL..16..748G. doi:10.1103/PhysRevLett.16.748.

[Zatsepin-2] Zatsepin, G. T.; Kuz'min, V. A. (1966). «Upper Limit of the Spectrum of Cosmic Rays». Journal of Experimental and Theoretical Physics Letters 4: 78–80. Bibcode:1966JETPL...4...78Z.

[Abbasi2008-3] Abbasi, R. U. (2008). «First Observation of the Greisen-Zatsepin-Kuzmin Suppression». Physical Review Letters 100 (10): 101101. Bibcode:2008PhRvL.100j1101A. PMID 18352170. arXiv:astro-ph/0703099. doi:10.1103/PhysRevLett.100.101101.

[Abraham2008-4] Abraham, J. (2008). «Observation of the suppression of the flux of cosmic rays above 4×10¹⁹ eV». Physical Review Letters 101 (6): 061101-1-061101-7. Bibcode:2008PhRvL.101f1101A. arXiv:0806.4302. doi:10.1103/PhysRevLett.101.061101.

[5] Ormes, Jonathan F. (2000). «The origin of cosmic rays: What can the Fermi Gamma-ray Telescope say?». AIP Conference Proceedings 528: 445–448. arXiv:astro-ph/0003270. doi:10.1063/1.1324357.

[Kalashev2007-6] Kalashev, Oleg E.; Semikoz, Dmitry V.; Sigl, Guenter (2007). «Ultra-High Energy Cosmic Rays and the GeV-TeV Diffuse Gamma-Ray Flux». Physical Review D 79 (6): 063005. Bibcode:2009PhRvD..79f3005K. arXiv:0704.2463. doi:10.1103/PhysRevD.79.063005.

[7] Fargion, D.; Mele, B.; Salis, A. (June 1999). «Ultra–High‐Energy Neutrino Scattering onto Relic Light Neutrinos in the Galactic Halo as a Possible Source of the Highest Energy Extragalactic Cosmic Rays». The Astrophysical Journal 517 (2): 725-733. Bibcode:1999ApJ...517..725F. arXiv:astro-ph/9710029. doi:10.1086/307203.

[8] The Pierre Auger Collaboration (2007). «Correlation of the Highest-Energy Cosmic Rays with Nearby Extragalactic Objects». Science 318 (5852): 938–943. Bibcode:2007Sci...318..938T. PMID 17991855. arXiv:0711.2256. doi:10.1126/science.1151124.

[9] The Pierre Auger Collaboration (2010). «Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter». Astropart. Phys. 34 (5): 314–326. Bibcode:2010APh....34..314A. arXiv:1009.1855. doi:10.1016/j.astropartphys.2010.08.010.

[10] The Pierre Auger Collaboration (2010). «Measurement of the energy spectrum of cosmic rays above 10¹⁸ eV using the Pierre Auger Observatory». Phys. Lett. B 685 (4–5): 239–246. Bibcode:2010PhLB..685..239A. arXiv:1002.1975. doi:10.1016/j.physletb.2010.02.013.

[11] Sokolsky; for the HiRes Collaboration (2010). «Final Results from the High Resolution Fly's Eye (HiRes) Experiment». arXiv:1010.2690 [astro-ph.HE].

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