Extra dimensions and strong neutrino-nucleon interactions above 10(19) eV:breaking the GZK barrier

Cosmic ray events above 10(20) eV are on the verge of confronting fundament al particle physics. The neutrino is the only candidate primary among estab lished particles capable of crossing 100 Mpc intergalactic distances unimpe ded. The magnitude of nu N cross sections indicated by events, plus consist ency with the Standard Model at low-energy, point to new physics of massive spin-2, exchange. In models based on extra dimensions, we find that the nu N cross section rises to typical hadronic values of between 1 and 100 mb a t energies above 1020 eV, Our calculations take into account constraints of unitarity. We conclude that air-showers observed with energies above 10(19 ) eV are consistent with neutrino primaries and extra-dimension models. An upper bound of 1-10 TeV on the mass scale at which graviton exchange become s strong in current Kaluza-Klein models follows, (C) 2000 Elsevier Science B.V, All rights reserved.