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In the Standard Model, gluons are massless spin-1 bosons just as photons are, so it stands to reason that they only have two possible eigenvalues of helicity for the same reason that photons, which are massless spin-1 bosons, have only two such values.

But, I could imagine that I am overlooking something important in this naive analysis, and would like to confirm that gluons like photons have only two helicity states. A previous question I asked seems to reach this conclusion, but with very little analysis or resort to sources, and I'm not sure I was specific enough in focusing on this point to clearly communicate this issue, so this question refers only to helicity.

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  • $\begingroup$ @DanielC Thank you for the edit. I appreciate your correction of my clerical error slip up. $\endgroup$
    – ohwilleke
    Commented Nov 28, 2017 at 22:31
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    $\begingroup$ What makes you think that the analysis made for a photon (which I hope you understood) may not be applicable to the gluon? $\endgroup$
    – DanielC
    Commented Nov 28, 2017 at 22:32
  • $\begingroup$ One concern is that, unlike photons, gluons are sometimes described as acquiring mass dynamically and I'm not sure that only "rest mass" is relevant to the analysis. See, e.g., O. Oliveira and P. Bicudo, "Running gluon mass from a Landau gauge lattice QCD propagator" (2011) J. Phys. G: Nucl. Part. Phys. 38 045003 doi:10.1088/0954-3899/38/4/045003. Another is that gluons have self-interactions that photons lack and a more complex charge structure which might be relevant. $\endgroup$
    – ohwilleke
    Commented Nov 28, 2017 at 22:35
  • $\begingroup$ According to the currently accepted theory (SM), the gluon is massless, yes, with "color charge", but this has no impact on the spacetime description (which follows from the Coleman-Mandula no-go theorem). Gluons cannot be directly observable, but they are +1 and -1 eigenstates of helicity. $\endgroup$
    – DanielC
    Commented Nov 28, 2017 at 22:38

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