Comparing the R_ξ gauge and the unitary gauge for the standard model: An example

For gauge theory, the matrix element for any physical process is independent of the gauge used. However, since this is a formal statement, it does not guarantee this gauge independence in every case. An example is given here where, for a physical process in the standard model, the matrix elements calculated with two different gauge – the $R_{\xi }$ gauge and the unitary gauge – are explicitly verified to be different. This is accomplished by subtracting one matrix element from the other. This non-zero difference turns out to have a subtle origin. Two simple operators are found not to commute with each other: in one gauge these two operations are carried out in one order, while in the other gauge these same two operations are carried out in the opposite order. Because of this result, a series of question are raised such that the answers to these question may lead to a deeper understanding of the Yang–Mills non-Abelian gauge theory in general and the standard model in particular.