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Below the strange-quark threshold, the gauge-field and two-flavour quark terms arewhereThis is the low-energy Quantum chromodynamics plus electromagnetism Lagrangian for the light flavours.

Every renormalizable gauge-invariant term contains equal numbers of quark and antiquark fields. The common phase symmetryis therefore automatic rather than imposed in constructing the Lagrangian. Its conserved charge is baryon number, making $U(1{)}_B$ an accidental symmetry of the renormalizable low-energy theory.

The strong-coupling scale $\Lambda$ is defined as the scale at which the one-loop inverse coupling vanishes. Setting $g^{-2}(\Lambda )=0$ givesEquivalently, in terms of the coupling measured at any scale $\mu$,This dimensional transmutation defines ${\Lambda }_{\mathrm{QCD}}$ for Quantum chromodynamics.

For $q=(u,d{)}^T$, massless two-flavour Quantum chromodynamics hasClassically its continuous global symmetry is $SU(2{)}_L\times SU(2{)}_R\times U(1{)}_V\times U(1{)}_A$, up to finite central quotients. The Adler-Bell-Jackiw anomaly destroys the continuous axial $U(1{)}_A$ in the quantum theory, leaving $SU(2{)}_L\times SU(2{)}_R\times U(1{)}_V$, together with an anomaly-preserved discrete axial subgroup.

The QCD scale is the renormalization-group invariant energy at which the perturbative running coupling becomes strong. It sets the characteristic scale of confinement and most hadron masses.

It is a characteristic energy scale of Quantum chromodynamics.

The strong coupling constant parametrizes the strength of Quantum chromodynamics interactions. Its dependence on the renormalization scale is governed by the renormalization-group beta function. It is dimensionless in four dimensions.

The strong interaction is the $SU(3{)}_c$ gauge interaction described by Quantum chromodynamics. It couples quarks and gluons through color charge.

With three massless lepton families, two accessible up-type quarks, three colors, and no Quantum chromodynamics radiative correction, a W boson has total decay width $9{\Gamma }_{\ell }$. With an identity CKM matrix, each leptonic channel has branching fraction $1/9$ and each of the two allowed quark-flavour channels has $1/3$. With mixing, each accessible quark channel is weighted by $|V_{ij}{|}^2$, while row unitarity preserves the total at this order if all down-type channels are accessible.