Can we born rule be used to find probability of a particle to exist in a region in QFT using the formula $\int_a^b \psi(x)\psi^*(x)dx$,where $\psi(x)$ is a fermionic field? If yes, please provide citations.
I cannot understand how can born rule be applied to a particle since a field contains multiple particle and all particles are explained by the plane wave solutions of the field.