Forward Neutrinos from Charm at Large Hadron Collider

The currently operating FASER experiment and the planned Forward Physics Facility (FPF) will detect a large number of neutrinos produced in proton-proton collisions at the LHC. In this work, we estimate neutrino fluxes at these detectors from charm meson decays, which will be particularly important for the $\nu_e$ and $\nu_\tau$ channels. We make prediction using both the next-to-leading order collinear factorization and the $k_T$-factorization approaches to model the production of charm quarks as well as different schemes to model their hadronization into charm hadrons. In particular, we emphasize that a sophisticated modeling of hadronization involving beam remnants is needed for predictions at FASER and FPF due to the sensitivity to the charm hadron production at low transverse momenta and very forward rapidity. As example, we use the string fragmentation approach implemented in \texttt{Pythia~8}. While both standard fragmentation functions and \texttt{Pythia~8} are able to describe LHCb data, we find that \texttt{Pythia~8} predicts significantly higher rate of high energy neutrinos, highlighting the importance of using the correct hadronization model when making predictions.