The W Boson Spin Density Matrix at OPAL

The spin states of W bosons in e<super>+</super>e<super>&minus;</super> &rarr; W<super>+</super>W<super>&minus;</super> &rarr; qq&macr;<super>&prime;</super>&ell;&nu;_&ell; events recorded by the OPAL detector at LEP are determined by an analysis of the angular distributions of their decay products. This information is used to calculate polarised differential cross-sections and to search for CP-violating effects. Results are presented for W bosons produced in e<super> +</super>e<super>&minus;</super> collisions with centre- of-mass energies between 183 GeV and 209 GeV. The average percentage of longitudinally polarised W bosons measured from the data is found to be (23.9 &plusmn; 2.1 &plusmn; 1.1)% compared to a Standard Model prediction of (23.9 &plusmn; 0.1)%. All results are consistent with CP conservation.