Measurement of the polarization parameter in π−p backward elastic scattering at 6 GeV/c

Abstract

The polarization parameter in π −p elastic scattering has been measured in the backward angular region at an incident momentum of 6 GeV/ c. The measurements cover the range of four momentum transfer u = 0 to −1 (GeV/ c) 2, and were obtained with a high intensity pion beam, a butanol polarized proton target, and arrays of scintillation counter hodoscopes. The polarization is different from zero, in contradiction to the prediction of the naive one trajectory Regge-exchange model. It increases positively with the four-momentum transfer u, reaching a maximum of about 0.4 at u ≈ −0.3 (GeV/ c) 2. It then decreases and becomes slightly negative beyond u ≈ −0.5 (GeV/ c) 2. A variety of baryon exchange models are briefly reviewed and none are found to be in complete agreement with all the experimental data.