Progress in the determination of the partonic structure of the proton

We review the state of the art of the determination of the parton substructure of the nucleon, as expressed in terms of parton distribution functions (PDFs) and probed in high-energy lepton–hadron and hadron–hadron collisions. We also assess their implications for current precision collider phenomenology, in particular at the Large Hadron Collider (LHC). We review the theoretical foundations of PDF determination: how cross sections are expressed in terms of PDFs by use of perturbative QCD factorization and evolution, the methodology used to extract PDFs from experimental data, and how different physical processes can be used to constrain different PDFs. We summarize current knowledge of PDFs and the limitations in accuracy that this knowledge currently entails for the computation of hadron collider processes, in particular at the LHC. We discuss the current main sources of theoretical and phenomenological uncertainties and the direction of progress toward their reduction in the future.