= Photon-gluon colour orthogonality
In distinct-flavour <quark> annihilation, the one-<photon> exchange <color charge> tensor is $\delta_{ji}\delta_{kl}$, whereas the one-<gluon> tensor is $\sum_AT^A_{ji}T^A_{kl}$. Their inner product after the external <color charge> sum is $\sum_A\operatorname{tr}(T^A)\operatorname{tr}(T^A)=0$, since the fundamental <gauge generators> of $SU(3)$ are traceless. Thus the averaged squared <scattering amplitude> has no leading electromagnetic–strong interference term for this process. Fixed external <color charge> components, or processes with different exchange pairings, need not share this cancellation.
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