  We study several sources of theoretical uncertainty in the determination of
parton distributions (PDFs) which may affect current PDF sets used for
precision physics at the Large Hadron Collider, and explain discrepancies
between them. We consider in particular the use of fixed-flavor versus
variable-flavor number renormalization schemes, higher twist corrections, and
nuclear corrections. We perform our study in the framework of the NNPDF2.3
global PDF determination, by quantifying in each case the impact of different
theoretical assumptions on the output PDFs. We also study in each case the
implications for benchmark cross sections at the LHC. We find that the impact
in a global fit of a fixed-flavor number scheme is substantial, the impact of
higher twists is negligible, and the impact of nuclear corrections is moderate
and circumscribed.
