Parton (particle physics)

It is useful for interpreting the cascades of radiation (a parton shower) produced from quantum chromodynamics (QCD) processes and interactions in high-energy particle collisions.

Unlike the uncharged photons, the gluons themselves carry colour charges and can therefore emit further radiation, leading to parton showers.

Thus, parton motion is slowed by time dilation, and the hadron charge distribution is Lorentz-contracted, so incoming particles will be scattered "instantaneously and incoherently".

[citation needed] Partons are defined with respect to a physical scale (as probed by the inverse of the momentum transfer).

[9] A parton distribution function (PDF) within so called collinear factorization is defined as the probability density for finding a particle with a certain longitudinal momentum fraction x at resolution scale Q2.

Parton distribution functions are obtained by fitting observables to experimental data; they cannot be calculated using perturbative QCD.

[14] They can be used to study the spin structure of the proton, in particular, the Ji sum rule relates the integral of GPDs to angular momentum carried by quarks and gluons.

They are accessed through a new class of exclusive processes for which all particles are detected in the final state, such as the deeply virtual Compton scattering.

The scattering particle only sees the valence partons. At higher energies, the scattering particles also detects the sea partons.
The CTEQ6 parton distribution functions in the MS renormalization scheme and Q = 2 GeV for gluons (red), up (green), down (blue), and strange (violet) quarks. Plotted is the product of longitudinal momentum fraction x and the distribution functions f versus x .