Charm production in the forward region: constraints on the small-x gluon and backgrounds for neutrino astronomy

Abstract: The recent observation by the IceCube experiment of cosmic neutrinos at energies up to a few PeV heralds the beginning of neutrino astronomy. At such high energies, the ‘conventional’ neutrino flux is suppressed and the ‘prompt’ component from charm meson decays is expected to become the dominant background to astrophysical neutrinos. Charm production at high energies is however theoretically uncertain, both since the scale uncertainties of the NLO calculation are large, and also because it is directly sensitive to the poorly-known gluon PDF at small-x. In this work we provide detailed perturbative QCD predictions for charm and bottom production in the forward region, and validate them by comparing with recent data from the LHCb experiment at 7 TeV. Finding good agreement between data and theory, we use the LHCb measurements to constrain the small-x gluon PDF, achieving a substantial reduction in its uncertainties. Using these improved PDFs, we provide predictions for charm and bottom production at LHCb at 13 TeV, as well as for the ratio of cross-sections between 13 and 7 TeV. The same calculations are used to compute the energy distribution of neutrinos from charm decays in pA collisions, a key ingredient towards achieving a theoretically robust estimate of charm-induced backgrounds at neutrino telescopes.