Everything about Perturbative Qcd totally explained
Perturbative QCD is a subfield of particle physics in which the theory of strong interactions,
Quantum Chromodynamics (QCD), is studied by using the factthat 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 we only consider the total production rate, 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't be calculated directly with
Perturbative QCD, since we in the real world doesn't have free
quarks and
gluons dute to
color confinement. We have to take
hadron structure into consideration, which involved
nonperturbative nature. Fortunately, physicist developed
QCD factorization theorem, which separate the
cross section into 2 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, we got 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 factors. There are several collaborations for each kind of universal long-distance functions. They have become an important part of modern
particle physics.
References and external links
Factorization of Hard Processes in QCD
(External Link
)
Further Information
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