Unitarity, the optical theorem, and the Pauli exclusion principle

Abstract

We show that the fermionic exclusion principle in scattering problems manifests itself through constraints implied by unitarity and the optical theorem. Configurations that formally allow identical fermions to appear in the same quantum state at the level of intermediate amplitudes are not pathological. Instead, they turn out to be essential for implementing the Pauli principle in scattering processes. Making this connection explicit resolves an apparent tension between the exclusion principle and unitarity and provides a clarified view of how fermionic statistics manifests itself within the $S$-matrix framework.