Related papers: Strongly Interacting Neutrinos as the Highest Ener…
Resonant photopion production with the cosmic microwave background predicts a suppression of extragalactic protons above the famous Greisen-Zatsepin-Kuzmin cutoff at about E_{GZK} ~ 5 x 10^10 GeV. Current cosmic ray data measured by the…
The spectrum of extra-galactic cosmic rays (CRs) is expected to follow the Greisen-Zatsepin-Kuzmin (GZK) cutoff at about 5x10^10 GeV which results from energy losses of charged nuclei in the cosmic microwave background. So far the…
Cosmic rays of energies larger than the Greisen-Zatsepin-Kuzmin (GZK) cutoff may be neutrinos if they acquire strong interactions due to a ``precocious unification'' of forces. A scenario for this to happen is outlined. There is no…
After a short review of the ultrahigh energy cosmic ray puzzle - the apparent observation of cosmic rays originating from cosmological distances with energies above the expected Greisen-Zatsepin-Kuzmin cutoff 4x10^{19} eV - we consider…
Contrary to earlier expectations, several cosmic ray events with energies above $10^{20}$ eV have been reported by a number of ultra-high energy cosmic ray observatories. According to the AGASA experiment, the flux of such events is well…
The cosmic ray spectrum has been shown to extend well beyond 10^20 eV. With nearly 20 events observed in the last 40 years, it is now established that particles are accelerated or produced in the universe with energy near 10^21 eV. In all…
The origin and nature of ultrahigh-energy cosmic ray events, above the Greisen-Zatsepin-Kuzmin (GZK) cutoff energy, constitute a long-standing, unsolved mistery. Neutrinos are proposed candidates but their standard interactions with matter…
The neutron fraction in the very high energy cosmic rays near the Greisen-Zatsepin-Kuzmin (GZK) cutoff energy is analyzed by taking into account the time dilation effect of the neutron decays and also the pion photoproduction behaviors…
The status of the Greisen-Zatsepin-Kuzmin (GZK) cutoff and pair-production dip in Ultra High Energy Cosmic Rays (UHECR) is discussed.They are the features in the spectrum of protons propagating through CMB radiation in extragalactic space,…
The origin and chemical composition of ultra high energy cosmic rays is still an open question in astroparticle physics. The observed large-scale isotropy and also direct composition measurements can be interpreted as an extragalactic…
Cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin `cutoff' at $\sim 4 \times 10^{10}$ GeV pose a conundrum, the solution of which requires either drastic revision of our astrophysical understanding, or new physics beyond the…
Cosmic ray events above 10^19 eV have posed a fundamental problem for more than thirty years. Recent measurements indicate that these events do not show the features predicted by the GZK bound. The events may, in addition, display angular…
Explanations of the origin of ultra-high energy cosmic rays are severely constrained by the Greisen-Zatsepin-Kuz'min effect, which limits their propagation over cosmological distances. We argue that possible departures from strict Lorentz…
Ultra-high energy cosmic rays (UHECRs) beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off provide us with a unique opportunity to understand the universe at extreme energies. Secondary GZK photons and GZK neutrinos associated with the same…
Ultra high energy cosmic rays (UHECR) pose a problem either for particle physics or for astrophysics (or for both) by the unexpectedly high number of cosmic ray showers observed with energy above about 5x10^{19}eV, the…
It has been suggested that the characteristic energy of string models may be considerably lower than the observed Planck mass. In such schemes, the unification of interactions takes place around the string scale, perhaps as low as a few…
The rarest cosmic rays above the GZK cut'off $(E_{CR} \tilde{>} 10^{19} \div 10^{20} eV)$ are probably born at cosmic distances ($\tilde{>}$ tens Mpc) by AGNs (QSRs, BLac, Blazars...). Their puzzling survival over $2.75 K^o$ BBR radio waves…
In the first part of this review we discuss the basic observational features at the end of the cosmic ray energy spectrum. We also present there the main characteristics of each of the experiments involved in the detection of these…
The clustering of ultra high energy cosmic ray events suggests that they have originated from compact sources. One of the possible physical mechanisms by which ultra high energy nuclei reach the Earth from far away astrophysical sources…
Special relativity has been tested at low energy with great accuracy, but its extrapolation to very high-energy phenomena is much less well established. Introducing a critical distance scale, a , below 10E-25 cm (the wavelength scale of the…