Relativistic bound-state mass from a rapidity pole
= Relativistic bound-state mass from a rapidity pole
{title2=$m_c^2=m_a^2+m_b^2+2m_am_b\cos u$}
At relative <rapidity> $iu$, the analytically continued sum of the constituent <four-momentum> vectors is on the bound-state <mass shell> with the displayed <mass>. For equal constituents, <rapidities> $\chi\pm iu/2$ sum to $2m_a\cos(u/2)(\cosh\chi,\sinh\chi)$. A <pole> at $u=0$ is at threshold rather than a strictly <bound state>.