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Neutrinos from charm production in the atmosphere

Atmospheric neutrinos are produced in interactions of cosmic rays with Earth's atmosphere. At very high energy, the contribution from semi-leptonic decays of charmed hadrons, known as the prompt neutrino flux, dominates over the conventional flux from pion and kaon decays. This is due to the very short lifetime of the charmed hadrons, which therefore do not lose energy before they decay. The calculation of this process is difficult because the Bjorken-x at which the parton distribution functions are evaluated is very small. This is a region where QCD is not well understood, and large logarithms must be resummed. Available parton distribution functions are not known at such small x and extrapolations must be made. Theoretically, the fast rise of the structure functions for small x ultimately leads to parton saturation. This contribution describes the "ERS" calculation of the prompt neutrino flux, which includes parton saturation effects in the QCD production cross section of charm quarks. The ERS flux calculation is used by e.g. the IceCube collaboration as a standard benchmark background. We are now updating this calculation to take into account the recent LHC data on the charm cross section, as well as recent theoretical developments in QCD. Some of the issues involved in this calculation are described.