Anomalous thresholds are a branch points in the complex plane of scattering amplitudes/Greens functions that are not easily deducible from unitarity. The book "The Analytic S-Matrix" by R.J. Eden explains it well, and shows how it may be deduced non-trivilaly from unitarity considerations.

Anomalous thresholds are branch points in the complex plane of scattering amplitudes/Greens functions that are not easily deducible from unitarity. The book "The Analytic S-Matrix" by R.J. Eden explains it well, and shows how it may be deduced non-trivilaly from unitarity considerations.