The three different helicity states of W+/- bosons, produced in the reactio
n e(+)e(-) --> W+W- --> lvq (q) over bar are studied using leptonic and had
ronic W decays at root s = 183 GeV and 189 GeV. The W polarisation is also
measured as a function of the scattering angle between the W- and the direc
tion 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 lepton
ic and hadronic W decays, the fraction of longitudinally polarised W+/- bos
ons is measured to be 0.261 +/- 0.016(syst.) +/- 0.016(syst.) in agreement
with the expectation from the Standard Model. (C) 2000 Elsevier Science B.V
. All rights reserved.