Parton model

The parton model describes hadrons at high energies as collections of pointlike constituents called partons. It explains how energetic probes scatter from quarks and gluons inside protons and other hadrons.

Bjorken scaling: The approximate independence of deep-inelastic scattering structure functions from momentum transfer at fixed scaling variable. Scaling suggested that the proton contains constituents behaving as nearly free pointlike scatterers.
Robert Hofstadter: An American physicist whose electron-scattering experiments measured the size and charge distribution of protons and nuclei. His work established proton structure before higher-energy experiments revealed pointlike constituents.
Hadron: A composite particle bound by the strong interaction, including protons, neutrons, and mesons. Partons are the constituents used to describe hadrons in high-energy collisions.
Feynman parton model: Richard Feynman's formulation of hadrons as fast-moving collections of constituents in high-energy collisions. This formulation made the parton picture concrete using the infinite-momentum frame.
Bjorken x: A dimensionless variable in deep-inelastic scattering that, in the parton model, approximates a struck parton's momentum fraction. It identifies the longitudinal momentum share assigned to the parton hit by the probe.
Stanford Linear Accelerator Center: A U.S. accelerator laboratory that began operating in 1962 and hosted landmark electron-scattering experiments. Its high-energy electron beams enabled the experiments that motivated the parton picture.
Proton: A positively charged baryon composed primarily of two up quarks and one down quark, bound by gluons. The proton was the principal target in the experiments that established the model.
Quark model: A classification model that organizes hadrons by their quark content and quantum numbers. It describes hadron spectroscopy, while the parton model addresses high-energy scattering.
Parton distribution function: A function describing the probability density for finding a parton with a specified momentum fraction inside a hadron. These distributions quantify the model's constituent content for cross-section predictions.
Jerome Friedman: An American physicist who shared the 1990 Nobel Prize for deep-inelastic electron-proton scattering experiments. He helped establish the experimental evidence for pointlike constituents in nucleons.

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