Unitarity implies branch cuts in $s 4m^2$ in the $S$-matrix

Question

Why does unitarity imply a branch cut in the $S$-matrix after $s > 4m^2$ where $s$ is the Mandlestam variable and $m$ is the mass of the particle? Assume identical particle scattering.

Arnold Neumaier · Answer

Because particle pairs produce a continuous spectrum with energy $(2m)^2+K$, where $Kge 0$ is the arbitrary relative relativistic kinetic energy.

Unitarity implies branch cuts in $s > 4m^2$ in the $S$-matrix

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