Insights into nucleon structure from parton distributions
2017 May 04
2017 May 09
We review recent progress in understanding the substructure of the nucleon from global QCD analysis of parton distribution functions (PDFs). New high-precision data on $W$-boson production in $p\bar p$ collisions have significantly reduced the uncertainty on the $d/u$ PDF ratio at large values of $x$, indirectly constraining models of the medium modification of bound nucleons. Drell-Yan data from $pp$ and $pd$ scattering reveal new information on the $\bar d-\bar u$ asymmetry, clarifying the role of chiral symmetry breaking in the nucleon. In the strange sector, a new chiral SU(3) analysis finds a valence-like component of the strange-quark PDF, giving rise to a nontrivial $s-\bar s$ asymmetry at moderate $x$ values. We also review recent analyses of charm in the nucleon, which have found conflicting indications of the size of the nonperturbative charm component.