Unitarity and crossing in Reggeon-particle amplitudes

We discuss the single Regge limit of the process $a+b\to c+d+e$, in which the five-point amplitude is proportional to the Reggeon-particle amplitude $a+R\to c+d$. The partial-wave expansion in the $c d$ system is given in the general case when all external particles have spin. Using this expansion we show that unitarity requires the phase of the Reggeon-particle amplitude to be the same as the phase of the amplitude for $c+d\to c+d$ in the elastic region. This implies relations between observable density matrix elements. We discuss the predictions for several physical reactions. We investigate the question of crossing for Reggeon-particle amplitudes by deriving the connection between the $s$-channel and $t$-channel helicity amplitudes. The helicity crossing matrix turns out to be the ordinary spin rotation matrix, the Reggeon being treated as a particle of continuous spin ${\alpha }_R$.