Measurement of the polarization parameter in $\pi^{-}p$ backward elastic scattering at 6 GeV/c

The polarization parameter in pi /sup -/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)/sup 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 approximately=-0.3(Ge V/c)/sup 2/. It then decreases and becomes slightly negative beyond u approximately=-0.5(GeV/c)/sup 2/. A variety of baryon exchange models are briefly reviewed and none are found to be in complete agreement with all the experimental data.