Parton Distribution Functions
  Parton distribution functions give the probability to find partons (quarks and gluons) in a hadron as a function of the fraction x of the proton's momentum carried by the parton. They are conventionally defined in terms of matrix elements of certain operators. They are determined from experimental results on short distance scattering of the partons. Integrals of these functions weighted with x^n are calculable using lattice QCD. Some simple models for their behavior have implications that could also be tested in lattice QCD. 
Davison E. Soper