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.
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$\begingroup$ this reads awfully like a homework question... $\endgroup$– AccidentalFourierTransformCommented Jul 24, 2020 at 6:56
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1$\begingroup$ related/possible dup? physics.stackexchange.com/q/289775/84967 $\endgroup$– AccidentalFourierTransformCommented Jul 24, 2020 at 6:56
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Because particle pairs produce a continuous spectrum with energy $(2m)^2+K$, where $K\ge 0$ is the arbitrary relative relativistic kinetic energy.
answered Jul 24, 2020 at 10:55