Faddeev–Popov ghost

Summary

Faddeev–Popov ghosts are auxiliary anticommuting fields introduced in the path integral of a gauge theory. Gauge fixing picks one field configuration from each gauge orbit, and it brings in a Jacobian determinant, the Faddeev–Popov determinant. Writing this determinant as a Berezin integral over a ghost field $c$ and an antighost field $\bar c$ turns it into a term of the action. Ghosts have no physical states of their own, but non-abelian gauge theories need them for correct results. They are the starting point of BRST quantization.