partonmodel

The parton model is a framework in high-energy physics for describing the internal structure of hadrons in terms of pointlike constituents, called partons. Proposed by Richard Feynman in the late 1960s to explain deep inelastic scattering, the model treats fast-moving hadrons as collections of quasi-free partons during the short interaction time. In the infinite momentum frame, the scattering cross section factorizes into a hard interaction of the probe with a single parton and a distribution of partons inside the hadron.

In deep inelastic scattering, experimental measurements showed scaling of the structure functions with the Bjorken variable x, consistent with partons behaving as free, pointlike constituents at high energies. For electromagnetic scattering, the Callan-Gross relation F2 = 2x F1 is predicted for spin-1/2 partons, aligning with quark expectations.

The parton model was later embedded within quantum chromodynamics (QCD) as its limiting framework. Partons are identified with quarks, antiquarks, and gluons. The probability to find a parton carrying a fraction x of the hadron’s momentum at a resolution scale μ^2 is described by parton distribution functions f_i(x, μ^2). Cross sections are obtained by convolving these PDFs with calculable partonic cross sections. QCD introduces scaling violations: the distributions evolve with Q^2 according to the DGLAP equations.

Parton distributions are constrained by data from deep inelastic scattering and hadron collider experiments, serving as essential inputs for predicting high-energy processes. Limitations include that partons are not free objects and confinement remains, with the parton model serving as an effective description valid at high energies and separated scales.