Cosmic Rays at the Highest Energies

Karl-Heinz Kampert
Bergische Universität Wuppertal

Recent data of giant air shower observatories, most importantly of the
3000 km^2 large Pierre Auger Observatory operated in Argentina, have
led to a number of major breakthroughs in the field of ultra-high
energy cosmic rays.  Most importantly, a distinct flux suppression
above 5∙10^19 eV has been has been established unambiguously. This,
together with upper limits on photon and neutrino fluxes at ultra-high
energy have ruled out particle physics motivated „top-down“ source
processes, such as the decay of super-heavy particles, to account for
a significant part to the observed particle flux at the highest
energies.  Moreover, there are indications for an anisotropic
distribution of the arrival directions of particles with energies
greater than 5∙10^19 eV. These results are typically considered as
strong support of source scenarios in which particle acceleration
takes place at sites distributed similarly to the matter distribution
in the universe, with energy loss processes in the CMB leading to the
observed flux suppression (GZK-effect). However, the cosmic ray mass
composition and the small level of anisotropies in arrival directions
suggest that the end of the cosmic ray spectrum is not dominated by
the GZK-effect but by an extragalactic source (or source population),
possibly within the GZK-horizon, running at its maximum
electromagnetic rigidity R=p/Z. In this talk, we shall review cosmic
ray data at the highest energies, discuss their implications to
understanding their origin, and we shall address some synergies to
particle physics currently performed at the LHC.