Neutrino Cross Sections at HERA and Beyond

The sources of very high energy cosmic rays should also be sources of neutrinos. While the complete scheme of ultrahigh energy cosmic ray production is not yet understood [1], ancillary to cosmic ray production are neutrino fluxes, both at the source of the cosmic rays and through cosmic ray propagation and scattering with the microwave background. Given that neutrinos are unabsorbed and undeflected between their sources and the Earth, neutrino telescopes may yield results that allow for a better understanding of cosmic ray production [2]. The neutrinonucleon cross section is a key ingredient in neutrino telescope measurements. The theoretical predictions of ultrahigh energy neutrino cross sections have been greatly influenced by the deep inelastic scattering (DIS) results of HERA experiments, starting in 1992. Neutrino cross sections were not measured at HERA, where beams of electrons and positrons are incident on proton beams, however, our knowledge of parton distribution functions has been dramatically improved with HERA results. At an energy equivalent to a neutrino energy of 54 TeV, the HERA results have aided our understanding of the parton distribution functions in new kinematic regimes and have led to different high energy extrapolations of the ultrahigh energy neutrino cross section than what was done earlier.