W polarisation and spin density matrix measurements at LEP

The helicity structure of the W-pair production is studied by the LEP experiments using either measurements of spin density matrix elements or a fit of the expected angular distribution of the different helicity states to the observed data. The measurements are in good agreement with the Standard Model expectation, clearly confirming the longitudinal component of the W-gauge-boson. The W-helicity is either observed for each W-boson individually or in correlation of the two bosons. The measurements also provide a test for genuine CP violation, with the measurements confirming CP invariance