Entanglement entropy of particles in a perturbative scattering

The entanglement among scattering particles in an exemplary quantum electrodynamics (QED) process is studied perturbatively. To increase the computational accuracy, we need to consider virtual photon loop diagrams, which lead to infrared divergence. Therefore, when including higher-order corrections in perturbative theory, we use the dressed state formalism proposed by Chuang and Faddeev-Kulish, which provides a finite S-matrix element. The entanglement entropy calculation shows that the entanglement is distributed linearly over the scattering cross section under the perturbation approximation