Astroparticle Physics

7.5 Nature and Origin of the Highest-Energy Cosmic Rays 167To obtain a reasonable interaction rate only neutrino interactionsfor inclined directions of incidence or in the Earthcan be considered. The resulting expected distribution of primaryvertices due to neutrino interactions is in contrast toobservation. Therefore, neutrinos as well can very likely beexcluded as candidates for the highest-energy cosmic airshowerevents.It has been demonstrated that a large fraction of matteris in the form of dark matter. A possible way out concerningthe question of high-energy particles in cosmic rayswould be to assume that weakly interacting massive particles(WIMPs) could also be responsible for the observedshowers with energies > 10 20 eV. It has to be consideredthat all these particles have only weak or even superweakinteractions so that their interaction rate can only be on theorder of magnitude of neutrino interactions.The events with energies exceeding 10 20 eV thereforerepresent a particle physics dilemma. One tends to assumethat protons are the favoured candidates. They must comefrom relative nearby distances (< 30 Mpc), because otherwisethey would lose energy by photoproduction processesand fall below the energy of 6 × 10 19 eV. It is, however,true that up to these distances there are quite a number ofgalaxies (e.g., M87). The fact that the observed events do notclearly point back to a nearby source can be explained by thefact that the extragalactic magnetic fields are so strong thatthe directional information can be lost, even if the protonsare coming from comparably close distances. Actually, thereare hints showing that these fields are more in the µGaussrather than in the nGauss region [6].Recent measurements, however, appear to indicate thatthe GZK cutoff might have been seen at least in the dataof the HiRes experiment (see Fig. 6.5). On the other hand,this finding is in conflict with results from the large AGASAair-shower array (see also the comment on page 83).Presently one assumes that in supernova explosions particlescan only be accelerated to energies of 10 15 eV byshock-wave mechanisms. At these energies the primary spectrumgets steeper (‘knee of the primary spectrum’). As alreadyshown, our Milky Way is too small to accelerate andstore particles with energies exceeding 10 20 eV. Furthermore,the arrival directions of the high-energy particles showpractically no correlation to the galactic plane. Therefore,one has to assume that they are of extragalactic origin.vertex distributionfor neutrinosWIMPsextragalactic magnetic fieldsacceleration mechanisms