Perturbative quantum chromodynamics

Perturbative quantum chromodynamics (also 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 ${\displaystyle \alpha _{s}}$ is small in high energy or short distance interactions, thus allowing perturbation theory techniques to be applied. In most circumstances, making testable predictions with QCD is extremely difficult, due to the infinite number of possible topologically-inequivalent interactions. Over short distances, the coupling is small enough that this infinite number of terms can be approximated accurately by a finite number of terms. Although only applicable at high energies, this approach has resulted in the most precise tests of QCD to date ^{[citation needed]}.

An important test of perturbative QCD is the measurement of the ratio of production rates for ${\displaystyle e^{+}e^{-}\to {\text{hadrons}}}$ and ${\displaystyle e^{+}e^{-}\to \mu ^{+}\mu ^{-}}$. Since only the total production rate is considered, the summation over all final-state hadrons cancels the dependence on specific hadron type, and this ratio can be calculated in perturbative QCD.

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^[who?] 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.