Direct observation of longitudinally polarised W± bosons

The three different helicity states of W± bosons, produced in the reaction e+e−→W+W−→ℓνqq̄ are studied using leptonic and hadronic W decays at s=183 GeV and 189 GeV. The W polarisation is also measured as a function of the scattering angle between the W− and the direction of the e− beam. The analysis demonstrates that W bosons are produced with all three helicities, the longitudinal and the two transverse states. Combining the results from the two center-of-mass energies and with leptonic and hadronic W decays, the fraction of longitudinally polarised W± bosons is measured to be 0.261 ± 0.051(stat.) ± 0.016(syst.) in agreement with the expectation from the Standard Model.