Correlations between polarisation states of W particles in the reaction e − e + → W − W + at LEP2 energies 189–209 GeV

In a study of the reaction e − e + → W − W + with the DELPHI detector, the probabilities of the two W particles occurring in the joint polarisation states transverse–transverse ( TT ), longitudinal–transverse plus transverse–longitudinal ( LT ) and longitudinal–longitudinal ( LL ) have been determined using the final states WW→lν qq̅ ( l = e , μ ). The two-particle joint polarisation probabilities, i.e. the spin density matrix elements ρ TT , ρ LT , ρ LL , are measured as functions of the W − production angle, θ _W^- , at an average reaction energy of 198.2 GeV. Averaged over all cosθ_W^- , the following joint probabilities are obtained: ρ̅_TT=(67±8)%, ρ̅_LT=(30±8)%, ρ̅_LL=(3±7)%. These results are in agreement with the Standard Model predictions of 63.0