Study of W boson polarisations and Triple Gauge boson Couplings in the reaction e+e- -> W+W- at LEP 2
Abstract
A determination of the single W Spin Density Matrix (SDM) elements in the reaction e+e- -> W+W- -> l nu q qbar (l=e/mu) is reported at centre-of-mass energies between 189 and 209 GeV. The data sample used corresponds to an integrated luminosity of 520 pb-1 taken by DELPHI between 1998 and 2000. The single W SDM elements, rhotau tau'W+- (tau,tau' = +/-1 or 0), are determined as a function of the W- production angle with respect to the e- beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Lambdatau tau', which assume the V-A coupling of the W boson to fermions. The measured SDM elements are used to obtain the fraction of longitudinally polarised Ws, with the result: sigmaL/sigmatot = 24.9 +/- 4.5(stat) +/- 2.2(syst) % at a mean energy of 198 GeV. The SDM elements are also used to determine the Triple Gauge Couplings Delta g1Z, Delta kappagamma, lambdagamma and g4Z, kappaZ and lambdaZ. For the CP-violating couplings the results of single parameter fits are: g4Z = -0.39 +0.19 -0.20 kappaZ = -0.09 +0.08 -0.05 lambdaZ = -0.08 +/- 0.07 . The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.