Study of W boson polarisations and Triple Gauge boson Couplings in the reaction e+e- -> W+W- at LEP 2

TL;DR

This paper measures W boson polarization states and triple gauge couplings in e+e- collisions at LEP 2, providing results consistent with the Standard Model and exploring potential CP-violating effects.

Contribution

It introduces a method to determine W boson polarization and gauge couplings using SDM elements from LEP 2 data, including CP-violating parameters.

Findings

Measured fraction of longitudinally polarized W bosons: 24.9%

Constraints on CP-violating couplings g_4^Z, kappa_Z, lambda_Z

Results align with Standard Model predictions

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, rho_{tau 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, Lambda_{tau 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: sigma_L/sigma_tot = 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 g_1^Z, Delta kappa_gamma, lambda_gamma and g_4^Z, kappa_Z and lambda_Z. For the CP-violating couplings the results of single parameter fits are: g_4^Z = -0.39 +0.19 -0.20 kappa_Z = -0.09 +0.08 -0.05 lambda_Z = -0.08 +/- 0.07 . The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.