A measurement of the polarization of Lambda from Z decays

The polarization of $\Lambda$ from Z decays have been studied with the {\sc Aleph} apparatus. For the first time evidence of longitudinal polarization of $s$-quarks from Z decay is observed. It is shown to agree with the prediction from the standard model and the naive quark model within an uncertainty of about 25\% due to experimental errors and the use of hadronization models in the prediction. The part of the prediction provided by hadronization models is controlled by measurements of the forward-backward asymmetry of $\Lambda$ and by the correlation between back-to-back $\Lambda \bar{\Lambda}$ pairs. Furthermore, transverse polarization has been measured along the normal to the plane spanned by the thrust axis and the $\Lambda$ direction, together with transverse spin--spin correlations normal to the event plane.