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→lνqq¯WW→lνqq¯ (l=e,μ). The two-particle joint polarisation probabilities, i.e. the spin density matrix elements ρTT, ρLT, ρLL, are measured as functions of the W− production angle, θW−θW− , at an average reaction energy of 198.2 GeV. Averaged over all cosθW−cos⁡θW− , the following joint probabilities are obtained:
ρ¯TT=(67±8)%,
ρ¯TT=(67±8)%,
ρ¯LT=(30±8)%,
ρ¯LT=(30±8)%,
ρ¯LL=(3±7)%.
ρ¯LL=(3±7)%.
These results are in agreement with the Standard Model predictions of 63.0%, 28.9% and 8.1%, respectively. The related polarisation cross-sections σTT, σLT and σLL are also presented.