Particle Candidates of Ultrahigh Energy Cosmic Rays

We discuss candidates for trans-GZK cosmic rays observed in a variety of detectors. Three types of primaries are represented among the abstracts submitted to this meeting: neutrin os causing a Z-burst, protons arising from the decay of ultra-heavy metastable particles and neutrinos within the framework of low scale string-like models of unification. We attempt to evaluate the relative merits of these schemes. No definite conclusion can be reached at this time. However, we point out that some schemes are more credible/predictive than others. Data to be gathered by the Pierre Auger observatories as well as orbiting detectors (OWL, Airwatch...) should be able to decide between the various schemes.Comment: 15 pages, LaTex. Substantially revised to take into account the discussion at HEP2001, Budapest July 200

Signatures of Precocious Unification in Orbiting Detectors

It has been conjectured that the string and unification scales may be substantially lower than previously believed, perhaps a few TeV. In scenarios of this type, orbiting detectors such OWL or AIRWATCH can observe spectacular phenomena at trans-GZK energies. We explore measurable signatures of the hypothesis that trans-GZK air showeres (``anomalous showers'') are originated by strongly interacting neutrinos. The results of a MC simulation of such air showers is described. A distinction between proton induced and ``anomalous'' showers becomes possible once a substantial sample of trans-GZK showers will be available.Comment: LaTeX 2e, 14 pages, 5 figures. Revised and expanded version: MC rerun, figures redrawn, text revised and expande

TeV String Theories, Mini Black Holes and Trans-GZK Cosmic Rays

We review the proposal that trans-GZK cosmic ray interactions are caused by neutrino primaries. The primaries cause excitations of strings and give rise to extensive air showers (EAS) resembling EAS induced by nuclei. We also show that in ``low scale'' string models (of characteristic energy about 70TeV) the excited string and the mini black hole pictures are equivalent.Comment: LaTeX 2e, 5 pages, no figures. Invited talk given at XIII ISVHECRI, Pylos, Greec