Ultra High Energy Cosmic Rays from earth-based observatories

The origin of the highest energy cosmic rays is one of the most exciting questions of astroparticle physics. Even though a general concept linking magnetic field and size of possible sources (the so called “Hillas plot” [1]) is the basis of our current understanding, up to now there are no generally accepted source candidates known to be able to produce particles of such extreme energies. At these energies cosmic rays are expected to exhibit a suppression in the energy spectrum because of their interaction with the microwave background radiation (CMB). This feature, known as the Greisen-Zatsepin-Kuz’min (GZK) effect [2], is at about ∼ 6 · 10 eV for protons. It limits the horizon from which these particles can be observed to a distance below about 100 Mpc (depending on the primary mass). The non-observation of the GZK effect in the data of the AGASA experiment [3] has motivated several theoretical and phenomenological models trying to explain the absence of the GZK effect. More recently both HiRes [4] and Auger [5] have shown evidence of a suppression such as expected from the GZK effect with high statistical significance. The recent observation of directional correlations of the most energetic Auger events with the positions of nearby Active Galactic Nuclei [6] complements the observation of the GZK effect very nicely. Mass composition is another important key to discriminate among different models about the origin of high-energy cosmic rays. Such measurements are difficult due to their strong dependence on hadronic interaction models. Only primary photons can be discriminated safely from protons and nuclei and recent upper limits to their flux largely constrain existing top-down models. In this paper, prepared for the Physics in Collisions 2008 Conference, each of these topics are exposed and reviewed.