Couchman, Jonathan P.; (2001) A measurement of the triple gauge boson couplings and W boson polarisation in W-pair production at LEP2. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Measurements of the trilinear gauge boson vertex coupling (TGC) pa-rameters and the W polarisation of W+W- boson pairs produced in e+e- collisions are presented. The data were recorded by the OPAL experiment at LEP during 1998, where a total integrated luminosity of 183 pb-1 was obtained at a centre-of-mass energy of 189 GeV. Only events where one W boson decays into a quark and an anti-quark, and the other W decays into a lepton and a neutrino are used. The selected data sample comprises of 1075 events. All the measurements are performed through a spin density matrix analysis of the W boson decay products. In addition to this, an analysis of the W production angle is performed to further constrain the measured trilinear gauge boson coupling parameters. The sets of TGC parameters measured are Aky-X-Agzi and kz-Yz-gz4l. The first set are all CP-conserving, where as the second set are all CP-violating. Both sets of parameters may have anomalous values without violating the SU(2)L X U(l)y gauge symmetry of the electroweak Lagrangian. The mea-sured values of the CP-conserving parameters are:Aky= -0.235+0.282 ; y = -0.0821+:0.102-0.095; agz1 = -0.038+0.102-0.095; and for the CP-violating parameters the following are obtained: kz = -0.184+g+0.091-0.065; yz = -0.136+0.161-0.194; gz4 = +0.0740+0.263-0.255. All errors include both statistical and systematic uncertainties. In addition to the measurement of the TGC parameters, the SDM analysis is used to directly measure the W bosons polarisation states. The W boson may have either longitudinal or transverse polarisation. The fraction of W bosons produced with longitudinal polarisation is found to be ol/ototal = (21.9 ± 3.4 ± 1.6)%, where the first error is statistical and the second systematic. The joint W-pair production fractions were found to be; 0TT/0total = (76.8 ± 9.0 ± 3.2)%, 0LL/0total = (20.6 ± 7.2 ± 1.8)% and oTL/ototal = (2.6 ± 14.7 ±3.8)%. All results are consistent with Standard Model expectations.

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