Measurement of Lambda polarization from Z decays

Abstract The polarizationof Λ baryons from Z decays is studied with the Alephapparatus. Evidence of longitudinal polarizationof s quarks from Z decay is observed for the first time. The measured longitudinal Λ polarizationis P L Λ = −0.32 ± 0.07 for z = p p beam > 0.3 . This agrees with the prediction of −0.39 ± 0.08 from the standard model and the constituent quarkmodel, where the error is due to uncertainties in the mechanism for Λ production. The observed Λ polarizationis diluted with respect to the primary s quark polarizationby Λ baryons without a primary s quark. Measurements of the Λ forward-backward asymmetry and of the correlation between back-to-back Λ Λ pairs are used to check this dilution. In addition the transverse Λ polarizationis 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 Λ direction.