The anticommuting fields $c$ and $\bar c$ represent the field-dependent <Faddeev-Popov determinant>. They are Lorentz-scalar <Grassmann fields>, appear only on internal lines, and contribute a minus sign for each closed <ghost loop>. Their diagrams cancel unphysical gauge-polarization contributions and are required for gauge-independent, unitary amplitudes in a non-Abelian covariant gauge.

Solution

Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 304 / 4 / d / ii / Solution

 Solution (source code)