Structure of tensor-exchange amplitudes and polarization ofπ−+p→η+n

The structure of amplitudes of ${\pi }^{-}p\to \eta n$ is investigated by two methods: (a) fixed-$t$ dispersion relations and effective Regge parametrization of the high-energy imaginary parts and (b) two Regge poles and finite-energy sum rules. In both cases the resulting imaginary part of the nonflip amplitude deviates from the dualabsorptive-model form. Similarities with the conclusions of other amplitude analyses involving tensor exchange are discussed. The question of $A_2-\rho$ exchange degeneracy is also investigated. An important outcome of the work is that both methods (a) and (b) require a ${\pi }^{-}p\to \eta n$ polarization of very small magnitude throughout.