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

Abstract

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®lnq[`(q)]WW{\rightarrow}l\nu q\bar{q} (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, qW-\theta _{W^{-}}, at an average reaction energy of 198.2GeV. Averaged over all cosqW-\cos\theta_{W^{-}}, the following joint probabilities are obtained:

[`(r)]TT=(67±8)%,\bar{\rho}_{TT}=(67\pm8)\%,

[`(r)]LT=(30±8)%,\bar{\rho}_{LT}=(30\pm8)\%,

[`(r)]LL=(3±7)%.\bar{\rho}_{LL}=(3\pm7)\%.

These results are in agreement with the Standard Model predictions of 63.0%, 28.9% and 8.1%, respectively. The related polarisation
cross-sections σ

TT
, σ

LT
and σ

LL
are also presented.