  We review the constraints on the small-x gluon PDF that can be derived by
exploiting the forward D meson production data from the LHCb experiment at
$\sqrt{s}=5,7$ and 13 TeV. We then discuss the phenomenological implications of
the resulting improved small-x gluon for ultra-high energy astrophysics, in
particular neutrino telescopes, as well as for the proposed Future Circular
Collider (FCC) with $\sqrt{s}=100$ TeV. We illustrate how at the FCC even
electroweak scale cross-sections can become sensitive to the small-x region of
the quark and gluon PDFs, and then demonstrate how the addition of the LHCb
heavy meson production measurements leads to a reduction of PDF uncertainties
for various benchmark cross-sections.

High Energy Physics - Phenomenology;High Energy Physics - Experiment