Triple-Regge Coupling and Diffractive Spin Dependence

From qualitative arguments based on Regge behavior of three-to-three scattering amplitudes, taken together with the assumption that the vacuum trajectory function passes through unity and has a finite slope there, we argue that (1) the vertex function coupling three vacuum trajectories vanishes linearly at zero momentum transfer, and (2) a certain constraint equation equivalent to the vanishing of a single crossed-channel helicity amplitude connects the $j+1\mathrm{}$ independent helicity amplitudes for forward diffractive scattering of a boson of spin $j\ge 1$. The appearance of certain spurious poles in the 3→3 amplitude plays an important role in the discussion. We check our arguments in a simple Feynman-diagram model.