Useful trick to compute correlation functions of composite operators

Abstract: In general, in gauge field theories, physical observables are represented by gauge-invariant composite operators, such as the electromagnetic current. As we recently demonstrated in the context of the $U\left(1\right)$ and $SU\left(2\right)$ Higgs models \cite{Dudal:2019pyg,Dudal:2020uwb,Maas:2020kda}, correlation functions of gauge-invariant operators exhibit very nice properties. Besides the well-known gauge independence, they do not present unphysical cuts, and their K\"{a}ll\'{e}n-Lehmann representations are positive, at least perturbatively. Despite all these interesting features, they are not employed as much as elementary fields, mainly due to the additional complexities involved in their computation and renormalization. In this article, we present a useful trick to compute loop corrections to correlation functions of composite operators. This trick consists of introducing an additional field with no dynamics, coupled to the composite operator of interest. By using this approach, we can employ the traditional algorithms used to compute correlation functions of elementary fields.