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 fundamental particle physics. The neutrino is the only candidate primary among established particles capable of crossing 100 Mpc intergalactic distances unimpeded. We argue that consistency with the Standard Model at low energies, together with large interactions at high energies, directly indicates new physics of massive spin-2 exchange. Models based on extra dimensions can give $\nu N$ cross sections orders of magnitude larger than Standard Model extrapolations in the ultra-high energy regime. The $\nu N$ cross section rises to between 0.1 mb to 10 mb, a range typical of hadronic cross sections, at energies above $10^{20}$ eV and depending on the unitarity-preserving model. We conclude that air showers as observed with energies above $10^{19}$ eV are consistent with neutrino primaries and extra-dimension models. The consistency requires an {\it upper bound} of 1-2 TeV on the scale at which graviton exchange becomes strong in current Kaluza-Klein models.