We provide an experimental and theoretical perspective on the behavior of
unpolarized distribution functions for the nucleon and pion on the
valence-quark domain; namely, Bjorken-x \gtrsim 0.4. This domain is key to much
of hadron physics; e.g., a hadron is defined by its flavor content and that is
a valence-quark property. Furthermore, its accurate parametrization is crucial
to the provision of reliable input for large collider experiments. We focus on
experimental extractions of distribution functions via electron and muon
inelastic scattering, and from Drell-Yan interactions; and on theoretical
treatments that emphasize an explanation of the distribution functions,
providing an overview of major contemporary approaches and issues.
Valence-quark physics is a compelling subject, which probes at the heart of our
understanding of the Standard Model. There are numerous outstanding and
unresolved challenges, which experiment and theory must confront. In connection
with experiment, we explain that an upgraded Jefferson Lab facility is
well-suited to provide new data on the nucleon, while a future electron ion
collider could provide essential new data for the mesons. There is also great
potential in using Drell-Yan interactions, at FNAL, J-PARC and GSI, to push
into the large-x domain for both mesons and nucleons. We argue furthermore that
explanation, in contrast to modeling and parametrization, requires a widespread
acceptance of the need to adapt theory: to the lessons learnt already from the
methods of nonperturbative quantum field theory; and a fuller exploitation of
those methods.Comment: Review article: 133 double-spaced pages, 44 figures, 6 table
