Study of W-boson polarisations and triple gauge boson couplings in the reaction e(+)e(-)-> W+W- at LEP 2

A determination of the single W spin density matrix (SDM) elements in the reaction e⁺e^-→W⁺W^-→lνqq (l=e/μ) 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, ρ_ττ'^W± (τ,τ'= ± 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, Λ_ττ', 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: σ {L}{σ} = 24.9 pm4.5( pm2.2({)\% at a mean energy of 198∈GeV. The SDM elements are also used to determine the triple gauge couplings Δg₁^Z,Δκ_γ,λ_γ and g₄^Z, ̃κ_Z and ̃λ {Z} . For the CP-violating couplings the results of single parameter fits are: g4Z = -0.39+0.19-0.20 ̃κZ = -0.09+0.08-0.05 ̃λ_Z = -0.08±0.07 The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model. © 2008 Springer-Verlag / Società Italiana di Fisica.