TITLE: Why don't high-energy experimentalists ever include Faddeev-Popov ghosts in their Feynman diagrams?
QUESTION [9 upvotes]: To correctly calculate scattering amplitudes in nonabelian gauge theory, one must include Feynman diagrams with internal Faddeev-Popov ghosts (fictitious fermionic scalars that only appear internally in loop diagrams, not as external legs). I've seen many different talks by high-energy experimentalists and particle phenomenologists, which have included plenty of complicated loop Feynman diagrams involving strong and weak interaction scattering processes (e.g. here and here). But I've never, ever seen any experimentalist ever show a Feynman diagram containing an internal FP ghost propagator. Why can they get away with neglecting the FP ghosts?

REPLY [6 votes]: Perturbative QCD computations can be done in ghost-free gauges, such as the axial gauge. As far as I understand, this is customary for the computations revolving around the parton density functions in the proton, or in the pion. As well as for the final state equivalent, the parton fragmentation functions. Combined, this makes for a rather large section of QCD phenomenology I think. This might be one reason explaining your experience.
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