Measurement of $\Lambda$ polarization from Z decays

The polarization of \Lambda baryons from Z decays is studied with the {\sc Aleph} apparatus. Evidence of longitudinal polarization of s quarks from Z decay is observed for the first time. The measured longitudinal \Lambda polarization is P^{\Lambda}_{L} = -0.32 \pm 0.07 for z = p/p_{\mathrm{beam}} > 0.3. This agrees with the prediction of -0.39 \pm 0.08 from the standard model and the constituent quark model, where the error is due to uncertainties in the mechanism for \Lambda production. The observed \Lambda polarization is diluted with respect to the primary s quark polarization by \Lambda baryons without a primary s quark. Measurements of the \Lambda forward-backward asymmetry and of the correlation between back-to-back \Lambda \bar{\Lambda} pairs are used to check this dilution. In addition the transverse \Lambda polarization is measured. An indication of transverse polarization, more than two standard deviations away from zero, is found along the normal to the plane defined by the thrust axis and the \Lambda direction. %