Direct observation of longitudinally polarised W± bosons

Abstract

The three different helicity states of W± bosons, produced in the reaction <math><mtext>e</mtext><msup><mi></mi><mn>+</mn></msup><mtext>e</mtext><msup><mi></mi><mn>−</mn></msup><mtext>→</mtext><mtext>W</mtext><msup><mi></mi><mn>+</mn></msup><mtext>W</mtext><msup><mi></mi><mn>−</mn></msup><mtext>→</mtext><mtext>ℓνq</mtext><mtext>q</mtext><mtext>̄</mtext></math> 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.