The Ultra High Energy Neutrino-Nucleon Cross Section

The ultra-high energy neutrino nucleon cross section grows at a surprising rate with energy due to QCD effects in the target. Recent electroproduction data allows an update of earlier predictions. We compare the results of our own calculations with those of other groups, and critically review the foundation and reliability of the calculations. The question of the `new physics' potential of neutrino telescopes sensitive to the total cross section in multi-PeV energy domain is considered. We point out a loophole in the arguments which might be an important consideration in extrapolating the cross section to extremely high energies.