Physical rapidity strip
= Physical rapidity strip
{title2=$0<\operatorname{Im}\vartheta<\pi$}
For relativistic two-body scattering in one spatial dimension, this strip is the standard domain containing bound-state and crossed-channel <poles>. A stable-particle interpretation requires appropriate <pole> kinematics and residues; not every singularity automatically represents a new particle. Crossing relates <rapidity> $\vartheta$ to $i\pi-\vartheta$.