Polarizations and cross sections of Lambda hyperons produced at backward angles in the reaction pi +12C--> Lambda +X at 4 GeV/c.

We have measured the polarizations and the differential cross sections of the produced /Lambda/'s at backward angles (70/degree//lt//theta//sub lab//lt/145/degree/) using a large-aperture spectrometer. The angular range is kinematically not accessible in interactions of the incident pions with free nucleons. The calculated polarizations in terms of superposition of an elementary process incorporating Fermi motion of nucleons in the nucleus are opposite in sign to the observed ones, while a quark-parton model of /Lambda/ production with a Thomas precession effect reproduces both the polarizations and the cross sections well.