Polarization ofΛ'sandΛ¯'sProduced by 400-GeV Protons

We have measured the polarization of 3×${10;}^6$ ${\Lambda }^0$ and 2.5×${10}^5$ ${\overline{\Lambda }}^0$ hyperons produced by 400-GeV protons on a beryllium target. The hyperons were detected at a fixed angle of 7.2 mrad and in the momentum range from 50 to 300 GeV/c for ${\Lambda }^0$ and 50 to 200 GeV/c for ${\Lambda }^0$. The ${\Lambda }^0$ polarization agrees with that measured at 24 and 300 GeV and is - 0.24 ± 0.04 at $p_T=2.1\mathrm{}$ GeV/c. The ${\overline{\Lambda }}^0$ polarization is zero up to $p_T=1.2\mathrm{}$ GeV/c.