Editing Gluon (section)

=== Group theory details ===
Formally, QCD is a [[gauge theory]] with [[SU(3)]] gauge symmetry. Quarks are introduced as [[spinor]]s in ''N''<sub>f</sub> [[flavour (particle physics)|flavor]]s, each in the [[fundamental representation]] (triplet, denoted '''3''') of the color gauge group, SU(3). The gluons are vectors in the [[Adjoint representation of a Lie group|adjoint representation]] (octets, denoted '''8''') of color SU(3). For a general [[lie group|gauge group]], the number of force-carriers, like photons or gluons, is always equal to the dimension of the adjoint representation. For the simple case of SU(''n''), the dimension of this representation is {{nowrap|{{itco|''n''}}<sup>2</sup> − 1}}.

In group theory, there are no color singlet gluons because [[quantum chromodynamics]] has an SU(3) rather than a [[U(N)|U(3)]] symmetry. There is no known [[A priori and a posteriori|''a priori'']] reason for one group to be preferred over the other, but as discussed above, the experimental evidence supports SU(3).<ref name="Griff"/> If the group were U(3), the ninth (colorless singlet) gluon would behave like a "second photon" and not like the other eight gluons.<ref>{{cite web |url=https://www.forbes.com/sites/startswithabang/2020/11/18/why-are-there-only-8-gluons/|title=Why Are There Only 8 Gluons?|website=[[Forbes]]}}</ref>