PROCEEDINGS What Can We Learn from Cosmic Rays?

Abstract: Ultra high energy cosmic rays (UHECR) pose a problem either for particle physics or for astrophysics (or for both) by the unexpectedly high number of cosmic ray showers observed with energy above 5£1019 eV, the Greisen-Zatsepin-Kuzmin (GZK) cuto ® [1]. Our emphasis is on those possible solutions of the puzzle which assume that ultra high energy neutrinos travel cosmic distances. We present, in detail, a model which is based on a low energy (50 to 100 TeV) transition to a higher than four dimensional string regime. Neutrino-quark cross sections grow exponentially close to the threshold of this new scale because of the fast increase of the density of string states and e®ectively acquire hadronic strength. 1