A high-energy hadron is treated as constituents scattering incoherently. In electromagnetic deep inelastic scattering, the charged constituents are quarks and antiquarks.
At leading order in the naive parton model, dimensionless deep-inelastic structure functions depend only on Bjorken x. Quantum chromodynamics evolution introduces scale dependence.
For massless spin-one-half partons at leading order, the unpolarized electromagnetic deep-inelastic structure functions obey this relation. Radiative Quantum chromodynamics corrections and target-mass effects can modify it.
The leading high-energy approximation neglects quark and target masses and transverse constituent momentum. A lightlike reference momentum is used for the hadron; this does not describe a massless particle at rest.
A density of partons with a given flavour and momentum fraction at a specified parton factorization scale. It is a number density, not generally a distribution normalized to one.
The scale at which collinear contributions are assigned to parton distribution functions rather than the hard scattering coefficient. The separate factors depend on this scale, while a consistently calculated physical differential scattering cross-section does not.
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