baryon contains three valence partons (quarks) and a meson contains two valence partons (a quark and an antiquark parton). At higher energies, however, observations show sea partons (nonvalence partons) in addition to valence partons. History The parton model was proposed by Richard Feynman in 1969, used originally for analysis of high-energy collisions. It was applied to electron/proton deep inelastic scattering by Bjorken and Paschos. Later, with the experimental observation of Bjorken scaling, the validation of the quark model, and the confirmation of asymptotic freedom in quantum chromodynamics, partons were matched to quarks and gluons. The parton model remains a justifiable approximation at high energies, and others have extended the theory over the years. It was recognized that partons describe the same objects now more commonly referred to as quarks and gluons. A more detailed presentation of the properties and physical theories pertaining indirectly to partons can be found under quarks. In 1994, partons were used by Leonard Susskind to model holography. Parton distribution functions A parton distribution function (PDF) within so called collinear factorization is defined as the probability density for finding a particle with a certain longitudinal momentum fraction x at resolution scale Q2. Because of the inherent non-perturbative nature of partons which cannot be observed as free particles, parton densities cannot be calculated using perturbative QCD. Within QCD one can, however, study variation of parton density with resolution scale provided by external probe. Such a scale is for instance provided by a virtual photon with virtuality Q2 or by a jet. The scale can be calculated from the energy and the momentum of the virtual photon or jet; the larger the momentum and energy, the smaller the resolution scale—this is a consequence of Heisenberg's uncertainty principle. The variation of parton density with resolution scale has been found to agree well with experiment; this is an important test of QCD. Parton distribution functions are obtained by fitting observables to experimental data; they cannot be calculated using perturbative QCD. Recently, it has been found that they can be calculated directly in lattice QCD using large-momentum effective field theory. Experimentally determined parton distribution functions are available from various groups worldwide. The major unpolarized data sets are: ABM by S. Alekhin, J. Bluemlein, S. Moch CTEQ, from the CTEQ Collaboration GRV/GJR, from M. Glück, P. Jimenez-Delgado, E. Reya, and A. Vogt HERA PDFs, by H1 and ZEUS collaborations from the Deutsches Elektronen-Synchrotron