Perturbative QCD
Encyclopedia
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
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 topologically inequivalent interactions possible. Over short distances, the coupling is small enough that this infinite number of terms can be approximated accurately by a much more manageable number of terms. Although limited in scope, this approach has resulted in the most precise tests of QCD to date.
The first and most clear test of Perturbative QCD is R measurement in annihilation. R is defined as the ratio of production rate for + → Hadrons to that for + → + . Since only the total production rate is considered, summation over all final state hadrons canceled the dependence on specific hadron type, and this R ratio can be calculated in Perturbative QCD strictly.
Unfortunately, most of the processes can not be calculated directly with Perturbative QCD, since one cannot observe free quarks and gluons due to color confinement - the hadron
structure has a nonperturbative nature. Fortunately, physicists developed the QCD factorization theorem, which separates the cross section
into two parts: the process dependent Perturbative QCD calculable short-distance parton
cross section, and the universal long-distance functions. Those universal long-distance functions can be measured with global fit to experiments. In such a way, scientists obtained a partly calculable prediction to particle
reaction processes.
The universal long-distance functions include the parton distribution functions, fragmentation functions, multi-parton correlation functions, generalized parton distribution, generalized distribution amplitude and many kinds of form factor
s. There are several collaborations for each kind of universal long-distance functions. They have become an important part of modern particle physics
.
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(QCD), is studied by using the fact that the strong coupling constant
is small in high energy or short distance interactions, thus allowing Perturbation theoryPerturbation theory (quantum mechanics)
In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. The idea is to start with a simple system for which a mathematical solution is known, and add an...
techniques to be applied. In most circumstances, making testable predictions with QCD is extremely difficult, due to the infinite number of topologically inequivalent interactions possible. Over short distances, the coupling is small enough that this infinite number of terms can be approximated accurately by a much more manageable number of terms. Although limited in scope, this approach has resulted in the most precise tests of QCD to date.
The first and most clear test of Perturbative QCD is R measurement in annihilation. R is defined as the ratio of production rate for + → Hadrons to that for + → + . Since only the total production rate is considered, summation over all final state hadrons canceled the dependence on specific hadron type, and this R ratio can be calculated in Perturbative QCD strictly.
Unfortunately, most of the processes can not be calculated directly with Perturbative QCD, since one cannot observe free quarks and gluons due to color confinement - the hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
structure has a nonperturbative nature. Fortunately, physicists developed the QCD factorization theorem, which separates the cross section
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....
into two parts: the process dependent Perturbative QCD calculable short-distance parton
Parton (particle physics)
In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions. It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons...
cross section, and the universal long-distance functions. Those universal long-distance functions can be measured with global fit to experiments. In such a way, scientists obtained a partly calculable prediction to particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
reaction processes.
The universal long-distance functions include the parton distribution functions, fragmentation functions, multi-parton correlation functions, generalized parton distribution, generalized distribution amplitude and many kinds of form factor
Form factor (QFT)
In effective field theory, a form factor is a function which gives the properties of a certain particle interaction without including all of the underlying physics...
s. There are several collaborations for each kind of universal long-distance functions. They have become an important part of modern particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
.