W boson polarisation at LEP2

Elements of the spin density matrixfor W bosonsin e(+)e(-) --> W+W- --> q (q) over barq'lv(l) events are measured from data recorded by the OPAL detector at LEP. This information is used to calculate polarised differential cross-sections and to search for CP-violatingeffects. Results are presented for W bosonsproduced in e+e- Collisions with centre-of-mass energies between 183 GeV and 209 GeV. The average fraction of W bosonsthat are longitudinally polarised is found to be (23.9 +/- 2.1 +/- 1.1)% compared to a Standard Model prediction of (23.9 +/- 0.1)%. All results are consistent with CP conservation. (C) 2004 Elsevier B.V. All rights reserved.