Polarization of Λ and anti Λ in 920 GeV fixed-target proton-nucleus collisions

A measurement of the polarization of Λ and anti Λ baryons produced in pC and pW collisions at √(s)=41.6 GeV has been performed with the HERA-B spectrometer. The measurements cover the kinematic range of 0.6 GeV/c<p perpendicular to <1.2 GeV/c in transverse momentum and -0.15 < xF < 0.01 in Feynman-x. The polarization results from the two different targets agree within the statistical error. In the combined data set, the largest deviation from zero, +0.054±0.029, is measured for xF<or sim -0.07. Zero polarization is expected at xF=0 in the absence of nuclear effects. The polarization results for the Lambda agree with a parametrization of previous measurements which were performed at positive xF values, where the Λ polarization is negative. Results of anti Λ polarization measurements are consistent with zero. (Orig.)