Cosmic rays

Cosmic rays (CR) are positive charged particles, mostly compound by atomic nuclei, from the lightest (~89% protons and ~10% helium/α-particles) to the heaviest elements (~1%). Due to the fact that they are charged, during its propagation in the outer space they are deflected by the galactic magnetic fields, making very difficult to identify its origin.

The spectrum of CR at Earth contains three general features: a knee above 1015 eV, an ankle around 3·1018 eV and a cutoff above 3·1019 eV. Below the knee the spectrum follow a power law E with α~2.7, above the knee the spectral index changes to α~3.0, to recover again α~2.6 above the ankle; finally, the index increases drastically α~5.0 when reaching the cutoff.

CR with energies below and around the knee are thought to be accelerated at galactic astrophysical objects like supernova remnants or powerful binary systems. In the case of the particles above the knee and below the ankle, the origin is not clear, but it should be some special and very efficient galactic accelerators. Finally, the particles above the ankle are thought to be from extragalactic origin, like active galactic nuclei (AGN) or gamma-ray bursts (GRB). We call Ultra High Energy CR (UHECR) to the last part of the spectrum, above the ankle, (above 1018 eV). Due to its very high energy and extragalactic origin, they are suitable for testing Quantum Gravity theories.

During the propagation of UHECR, the adiabatic energy loss due to the expansion of the universe in always present, but in adittion, we should consider two main processes for protons, the pion photoproduction (p+γ→π0+p and p+γ→π++n) and the Bethe-Heitler pair production (p+γ→e-+e++p). In the case of heavy nuclei, they lose energy due to photodisintegration (A+γ→(A-1)+N).

Let us notice that charged pions will decay quickly to muons (π+→μ++νμ) and the muon will decay to a positron and neutrinos (μ+→e++νe+νμ). This will originate a secondary flux of neutrinos accompanying the CR called the flux of cosmogenic neutrinos or GZK neutrinos, which are the neutrinos which stem from the interaction of the CR with the background photons, in contrast from the usual flux of neutrinos that are supposed to originate on neutrino sources.

The pion photoproduction has an energy threshold of ~7·1019 eV, if considering CMB photons of around 10-3 eV, this way, protons with energy above this threshold will undergo pion photoproduction and will lose energy until they fall below the threshold. This is called the GZK cutoff, and it is of the same order of magnitude as the experimentally detected cutoff; although some detections appear to have exceeded the limit. Multiple explanations has been studied for this inconsistency, like considering more percentage of heavy nuclei in the composition of UHECR, or proposals that involve new physics.