Pion-Proton Interaction at 4.13 BeV/c and Backward Scattering

The experimental data for ${\pi }^{-}-p$ scattering at 4.13 BeV/c are analyzed. The main results are the following: (i) There is an upper limit for the backward peak owing to the unitarity of $S$ matrix. (ii) In spite of the fact that the total elastic cross section comes almost entirely from the forward peak, the $S$ matrix for the partial wave is markedly affected by the existence of a small backward peak.