I give a brief review of the parton model. The parton model pictures hadrons as a collection of pointlike quasi-free particles. The model describes the cross section for high-energy scattering of hadrons with another particle as an incoherent sum of the cross sections of the pointlike partons in the hadron with the other particle. The hadronic factors in the cross sections are parametrized by “structure functions. ” The parton model expresses the structure functions in terms of parton distribution functions that give the longitudinal momentum distribution of the partons in the given hadron. The parton distribution functions are found from experimental data in a given process and are used in the description of other processes. The prototype process for the parton model is eN → e ′ X, where e and e ′ are the incident and scattered electron, N is the target nucleon, and X is the set of final state hadrons. The particles in the final state X are not measured, so the cross section is for the sum over all hadronic final states, an “inclusive ” cross section. This contrasts with an “exclusive ” cross section in which the final states are restricted to a specific subset. In the prototype process, eN → e ′ X, the kinematics of the inclusive scattering depends on the momentum transfer q = k − k ′ from the electron to the hadrons and the invariant mass, W, of the hadronic final state, where W 2 = (p + q) 2 = M2 + 2Mν + q2, and M is the mass of the target nucleon or other hadron. Here k and k ′ are the energy-momentum 4-vectors of the incident
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