A measurement of Λ and Ξ− polarization in inclusive production by Σ− of 340 GeV/c in C and Cu targets

We have measured the polarization of Λ and Ξ− hyperons produced inclusively by a Σ− beam of 340 GeV/c momentum in nuclear targets, using samples of 9.5 million Λ decays and 880000 Ξ− decays. The large statistics allowed a two-dimensional analysis as functions of xF and pt. The polarization w.r.t. the production normal is mainly positive for Λ and negative for Ξ−. The positive sign of the Λ polarization is opposite to predictions. At pt>1 Gev/c, the Λ polarization decreases to zero or even negative values, in sharp contrast to the plateau above pt>1 Gev/c observed in inclusive Λ production by protons. No such effect is observed for the Ξ− polarization, and its sign is as expected.