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Hence, the actual value of the coupling constant is only defined at a given energy scale. In QCD, the Z boson mass scale is typically chosen, providing a value of the strong coupling constant of α s (M Z 2) = 0.1179 ± 0.0010. [7] In 2023 Atlas measured α s (M Z 2) = 0.1183 ± 0.0009 the most precise so far.
The strong force is the expression of the gluon interaction with other quark and gluon particles. All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parameterized by the strong coupling constant.
For quantum chromodynamics, the constant changes with respect to the distance between the particles. This phenomenon is known as asymptotic freedom. Forces which have a coupling constant greater than 1 are said to be "strongly coupled" while those with constants less than 1 are said to be "weakly coupled." [7]
The value of the fine-structure constant α is linked to the observed value of this coupling associated with the energy scale of the electron mass: the electron's mass gives a lower bound for this energy scale, because it (and the positron) is the lightest charged object whose quantum loops can contribute to the running.
(It is a non-trivial result of quantum field theory [2] that the exchange of even-spin bosons like the pion (spin 0, Yukawa force) or the graviton (spin 2, gravity) results in forces always attractive, while odd-spin bosons like the gluons (spin 1, strong interaction), the photon (spin 1, electromagnetic force) or the rho meson (spin 1, Yukawa ...
g s is the coupling constant of the strong force; t a = λ a /2 are the Gell-Mann matrices λ a divided by 2; a is a color index in the adjoint representation of SU(3) which take values 1, 2, ..., 8 for the eight generators of the group, namely the Gell-Mann matrices; μ is a spacetime index, 0 for timelike components and 1, 2, 3 for spacelike ...
The force between quarks is known as the colour force [6] (or color force [7]) or strong interaction, and is responsible for the nuclear force. Since the theory of electric charge is dubbed " electrodynamics ", the Greek word χρῶμα ( chrōma , "color") is applied to the theory of color charge, "chromodynamics".
The strong coupling constant is conventionally labelled g s (or simply g where there is no ambiguity). The observations leading to the discovery of this part of the Standard Model are discussed in the article in quantum chromodynamics .