Perturbative QCD is a subfield of particle physics in which the theory of strong interactions, Quantum Chromodynamics (QCD), is studied by using the fact that the strong coupling constant
An important test of perturbative QCD is the measurement of the ratio of production rates for
Most strong-interaction processes can not be calculated directly with perturbative QCD, since one cannot observe free quarks and gluons due to color confinement. For example, the structure hadrons has a non-perturbative nature. To account for this, physicists developed the QCD factorization theorem, which separates the cross section into two parts: the process dependent perturbatively-calculable short-distance parton cross section, and the universal long-distance functions. These universal long-distance functions can be measured with global fit to experiments and include parton distribution functions, fragmentation functions, multi-parton correlation functions, generalized parton distributions, generalized distribution amplitudes and many kinds of form factors. There are several collaborations for each kind of universal long-distance functions. They have become an important part of modern particle physics.