Parton distribution function

A parton distribution function gives the probability density for finding a quark or gluon carrying a specified fraction of a hadron’s longitudinal momentum, at a stated energy scale.

Deep inelastic scattering: A process in which a high-energy lepton probes a hadron through an exchange with large momentum transfer. Its measured cross sections reveal the momentum distributions of quarks inside nucleons.
Parton: A collective name for quarks and gluons treated as constituents of hadrons in high-energy processes. These are the constituents whose momentum fractions the distribution describes.
Parton distribution function sets: Numerical collections of fitted parton distributions with uncertainties, provided at specified scales and conventions. These sets supply the inputs used in practical cross-section calculations.
Feynman parton model: A model treating a fast hadron as a collection of nearly independent constituents during a high-energy collision. It introduced the constituent picture from which momentum distributions developed.
Small-x physics: The study of partons carrying very small fractions of a hadron’s longitudinal momentum. Measurements and theory remain challenging where gluon densities grow rapidly at small momentum fractions.
Dokshitzer–Gribov–Lipatov–Altarelli–Parisi equations: Evolution equations describing how parton distributions change with the energy scale of a probe. They predict how a distribution measured at one scale changes at another.
Hadron: A composite particle held together by the strong interaction, including protons and neutrons. The distribution describes the partonic content of this composite object.
Hadron collider: A particle accelerator that collides hadrons, such as protons, at high energies. Its event rates depend on the distributions of partons inside the colliding hadrons.
Richard Feynman: An American theoretical physicist who introduced the parton model in the 1960s. His model supplied the name and initial physical picture behind the distributions.
Color glass condensate: A high-density effective description of small-momentum-fraction gluon fields in fast hadrons or nuclei. It offers a nonlinear framework where ordinary dilute parton evolution may fail.

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