partons

Partons are the effective degrees of freedom used to describe the internal structure of hadrons in high-energy processes. The concept emerged in the parton model proposed by Richard Feynman in the late 1960s to explain deep inelastic scattering. In this framework a fast-moving hadron behaves as a cloud of quasi-free constituents, whose interactions during the collision are suppressed. These constituents are quarks, antiquarks, and gluons.

In quantum chromodynamics, partons correspond to the quarks and gluons that carry the hadron’s color charge content. They are not observed as free particles; instead, their distributions inside a hadron are encoded in parton distribution functions, PDFs. A PDF gives the probability to find a parton of type i carrying a fraction x of the hadron’s momentum when probed at a momentum transfer scale Q^2.

The scale dependence of partons is described by the DGLAP evolution equations, reflecting the increasing resolution at higher Q^2 and the associated emission and absorption of gluons. Early deep inelastic scattering data showed approximate scaling, which QCD explains as logarithmic violations of scaling with Q^2.

The parton content includes valence quarks, sea quarks, and gluons. Sum rules constrain PDFs, such as the momentum sum rule requiring that the total momentum carried by all partons equals the hadron’s momentum. Parts of the spin decomposition and the role of gluons are active areas of study.

In practice, PDFs are extracted from experimental data and used to predict cross sections in high-energy processes, including jet production and hadron collisions. Partons therefore provide a practical, gauge-invariant description of hadron structure within quantum chromodynamics.

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