Very high energy cosmic rays and search for physics beyond the standard model

The origin of highest energy cosmic rays (UHECR) is yet unknown. In order to understand their propagation we determine the probability that an ultrahigh energy (above 5\cdot 10^{19} eV) proton created at a distance r with energy E arrives at earth above a threshold E_c. The clustering of ultrahigh energy cosmic rays suggests that they might be emitted by compact sources. A statistical analysis on the source density based on the multiplicities is presented. The ultrahigh energy cosmic ray spectrum is consistent with the decay of GUT scale particles. Alternatively, we consider the possibility that a large fraction of the ultrahigh energy cosmic rays are decay products of Z bosons which were produced in the scattering of ultrahigh energy cosmic neutrinos on cosmological relic neutrinos. Based on this scenario we determine the required mass of the heaviest relic neutrino. The required ultrahigh energy neutrino flux should be detected in the near future by experiments such as AMANDA, RICE or the Pierre Auger Observatory.