Measurement of b-baryon polarization in $Z^{0}$ decays

In the Standard Model, b quarks produced in e/sup +/e/sup -/ annihilation at the Z/sup 0/ peak have a large average longitudinal polarization of -0.94. Some fraction of this polarization is expected to be transferred to b-flavored baryons during hadronization. The average longitudinal polarization of weakly decaying b baryons, <P /sub L//sup lambda b/>, is measured in approximately 4.3 million hadronic Z/sup 0/ decays collected with the OPAL detector between 1990 and 1995 at LEP. Those b baryons that decay semileptonically and produce a lambda baryon are identified through the correlation of the baryon number of the h and the electric charge of the lepton. In this semileptonic decay, the ratio of the neutrino energy to the lepton energy is a sensitive polarization observable. The neutrino energy is estimated using missing energy measurements. From a fit to the distribution of this ratio, the value <P/sub L//sup lambda b/>=-0.56 /sub -0.13//sup +0.20/+or-0.09 is obtained, where the first error is statistical and the second systematic. (21 refs).