  We discuss the sensitivity of theoretical predictions of observables used in
searches for new physics to parton distributions (PDFs) at large momentum
fraction $x$. Specifically, we consider the neutral-current Drell-Yan
production of gauge bosons with invariant masses in the TeV range, for which
the forward-backward asymmetry of charged leptons from the decay of the gauge
boson in its rest frame is a traditional probe of new physics. We show that the
qualitative behaviour of the asymmetry depends strongly on the assumptions made
in determining the underlying PDFs. We discuss and compare the large-$x$
behaviour of various different PDF sets, and find that they differ
significantly. Consequently, the shape of the asymmetry observed at lower
dilepton invariant masses, where all PDF sets are in reasonable agreement
because of the presence of experimental constraints, is not necessarily
reproduced at large masses where the PDFs are mostly unconstrained by data. It
follows that the shape of the asymmetry at high masses may depend on
assumptions made in the PDF parametrization, and thus deviations from the
traditionally expected behaviour cannot be taken as a reliable indication of
new physics. We demonstrate that forward-backward asymmetry measurements could
help in constraining PDFs at large $x$ and discuss the accuracy that would be
required to disentangle the effects of new physics from uncertainties in the
PDFs in this region.
