Hermitian analyticity of a two-particle S-matrix
= Hermitian analyticity of a two-particle S-matrix
In a real species basis, the two-particle <S-matrix> satisfies $S(-\theta^*)=S(\theta)^\dagger$ under the usual scattering analyticity assumptions. On the real <rapidity> axis this relates inverse-rapidity amplitudes to <complex conjugates>. Combined with the algebraic inverse relation $S(\theta)S(-\theta)=I$, it gives physical <unitarity>.