  We revisit the calculation of charm particle production in hadron collisions,
focusing on the production of charm particles that carry a large fraction of
the momentum of the incident proton. In the case of strange particles, such a
component is familiar from the abundant production of $K^+\Lambda$ pairs.
Modern collider experiments have no coverage in the very large rapidity region
where the forward pair production dominates. While forward charm particles are
produced inside the LHC beampipe, they dominate the high-energy atmospheric
neutrino flux in underground experiments because long-lived pions and kaons
interact before decaying into neutrinos. The fragmentation of the spectator
quark in the partonic subprocesses $qc \rightarrow qc$ and $gc \rightarrow gc$
is responsible for the forward component of charm production in perturbative
QCD. We use this phenomenological framework to construct a charm cross section
that saturates available accelerator and cosmic ray data, i.e., it represents
an upper limit on the normalization of the charm cross section that cannot be
reliably calculated because the charm mass is much smaller than the
center-of-mass energy. Where the highest energy IceCube observations are
concerned, we conclude that the upper limit on the flux of neutrinos from
forward charm production may dominate the much-studied central component. It
may therefore also represent a significant contribution to the TeV atmospheric
neutrino flux but cannot accommodate the PeV flux of high-energy cosmic
neutrinos observed by IceCube, or even the excess of events observed in the 30
TeV energy range.
