https://doi.org/10.1140/epjp/s13360-021-02273-3
Tutorial
Using Covariant Polarisation Sums in QCD
Institutt for fysikk, NTNU, Trondheim, Norway
Received:
10
September
2021
Accepted:
6
December
2021
Published online:
4
January
2022
Covariant gauges lead to spurious, non-physical polarisation states of gauge bosons. In QED, the use of the Feynman gauge, 
, is justified by the Ward identity which ensures that the contributions of non-physical polarisation states cancel in physical observables. In contrast, the same replacement can be applied only to a single external gauge boson in squared amplitudes of non-abelian gauge theories like QCD. In general, the use of this replacement requires to include external Faddeev–Popov ghosts. We present a pedagogical derivation of these ghost contributions applying the optical theorem and the Cutkosky cutting rules. We find that the resulting cross terms 
between ghost amplitudes cannot be transformed into 
in the case of more than two ghosts. Thus the Feynman rule stated in the literature holds only for two external ghosts, while it is in general incorrect.
© The Author(s) 2021
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