Entanglement between quantum fields
We analyze entanglement between quantum interacting fields. In particular, we use R\'enyi entropy to quantify the entanglement between the fields in the ground state of the linear $\sigma$ model. We adopt R\'enyi entropy because the failure of the replica trick method does not permit to obtain the Von Neumann entropy. We find that the degree of entanglement is larger in the spontaneously broken symmetry case and a generalization of geometric entropy area law is obtained. Finally, we suggest that entanglement between fields can be used as a signature of the presence of unobserved fields, such as is the Higgs model case with its unobserved sector.