  IceCube, a cubic kilometer neutrino telescope will be capable of probing
neutrino-nucleon interactions in the ultrahigh energy regime, far beyond the
energies reached by colliders. In this article we introduce a new observable
that combines several advantages: it only makes use of the upward-going
neutrino flux, so that the Earth filters the atmospheric muons, and it is only
weakly dependent on the initial astrophysical flux uncertainties.
