  Scenarios with new physics particles feebly interacting with the Standard
Model sector provide compelling candidates for dark matter searches. Geared
with a set of new experiments for the detection of neutrinos and long-lived
particles the Large Hadron Collider (LHC) has joined the hunt for these elusive
states. On the theoretical side, this emerging physics program requires
reliable estimates of the associated particle fluxes, in particular those
arising from heavy hadron decays. In this work, we provide state-of-the-art QCD
predictions for heavy hadron production including radiative corrections at
next-to-leading order and using parton distribution functions including
small-$x$ resummation at next-to-leading logarithmic accuracy. We match our
predictions to parton showers to provide a realistic description of
hadronisation effects. We demonstrate the utility of our predictions by
presenting the energy spectrum of neutrinos from charm hadron decays.
Furthermore, we employ our predictions to estimate, for the first time, FASER's
sensitivity to electrophilic ALPs, which are predominantly generated in beauty
hadron decays.
