Faddeev-Popov ghost

In physics, Faddeev-Popov ghost cⁱ is a field that violates the spin-statistics relation. It is the most essential new tool for BRST quantization. Anticommuting ghosts are associated with bosonic symmetries, while commuting ghosts are associated with fermionic symmetries.

The Faddeev-Popov ghosts are sometimes referred to as "good ghosts". The "bad ghosts" represent another, more general meaning of the word "ghost" in theoretical physics: states of negative norm – or fields with the wrong sign of the kinetic term – whose existence allows the probabilities to be negative.