The scattering of positive 120 MeV pions on protons

An investigation of the elastic scattering of 120 MeV positive pions by protons is described. The experiment was done by exposing a liquid propane chamber to the CERN 600 MeV synchro-cyclotron. The results refer to 5405 selected events in which the contamination from scattering on carbon nuclei is shown to be negligible. The values obtained for the phase-shifts are: α31= − 2.600±0.690, α3 = − 11.050±1.320, α33 = 31.670± 1.010. The value of α3 differs significantly from that expected if the linear dependence of α3 on the momentum, which has been proposed by some authors, is assumed. The present result is discussed, particularly in connection with some recent theoretical developments.