Griffiths Introduction to Elementary Particles · Problem 9.1

Quantum Chromodynamics — David J. Griffiths

Physics — QFT · ninth-gluon-photon-unification

Problem

What serious difficulty arises if one proposes that the ``ninth gluon'' (the color singlet

|9\rangle = (r\bar{r} + b\bar{b} + g\bar{g})/\sqrt{3}

) is identified with the photon? (See the discussion in Section 9.1.)