parton

Parton is a term used in quantum chromodynamics to denote the effective constituents of hadrons when probed at high momentum transfer. The concept originated with the parton model introduced by Richard Feynman in 1969 to explain the approximate scaling behavior seen in deep inelastic scattering experiments. In contemporary QCD, partons are quarks, antiquarks, and gluons—the fundamental degrees of freedom carrying color charge. They are not observed as free particles due to confinement; instead, they are internal components described by parton distribution functions (PDFs), which give the probability density to find a parton carrying a fraction x of the hadron's longitudinal momentum at a given resolution scale Q^2.

In practical calculations for high-energy processes, factorization theorems separate the long-distance physics encoded in PDFs from the short-distance dynamics calculable in perturbative QCD. The PDFs depend on x and Q^2 and evolve with Q^2 according to the DGLAP equations. Measurements from deep inelastic scattering and collider experiments constrain PDFs for quarks, antiquarks, and gluons, enabling predictions for cross sections at colliders such as the LHC.

The term "parton" emphasizes the role of these constituents in high-energy scattering; it is not a fixed particle species. Quarks and gluons are confined within hadrons at low energies, but behave as quasi-free degrees of freedom at high energies.

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