colorsinglet

Colorsinglet, or color singlet, refers to a representation of the SU(3) color gauge group in quantum chromodynamics (QCD) whose color state is invariant under color rotations. Mathematically, a color-singlet state |ψ> satisfies T^a |ψ> = 0 for all eight generators T^a of SU(3). Equivalently, its color part transforms in the trivial, or singlet, representation.

In hadrons, color singlets are essential because observable particles are color-neutral due to confinement. Quarks must combine into colorless configurations: mesons as a quark and an antiquark with their colors contracted (for example, a color-anticolor pair), and baryons as three quarks combined with an antisymmetric color wavefunction ε_{abc} to form a singlet. In representation terms, 3 ⊗ 3̄ = 1 ⊕ 8 and 3 ⊗ 3 ⊗ 3 contains a singlet, corresponding to baryons. The color content of hadrons is not directly observable; only color-singlet states appear as asymptotic, physical states.

Color-singlet operators are gauge-invariant objects used in theory and lattice QCD to project onto color-neutral states. Examples include quark bilinears q̄ Γ q and the three-quark operator ε_{abc} q^a q^b q^c. Lattice QCD computes hadron properties from correlators of such color-singlet operators.

In phenomenology, color-singlet concepts appear in production mechanisms of heavy quarkonia, where both color-singlet and color-octet intermediate states can contribute before hadronization into a singlet state.