Measurement of W polarisation at LEP

Abstract

The three different helicity states of W bosons produced in the reaction e+e-→lvqq at LEP are studied using leptonic and hadronic W decays. Data at centre-of-mass energies √s=183–209 GeV are used to measure the polarisation of W bosons, and its dependence on the W boson production angle. The fraction of longitudinally polarised W bosons is measured to be 0.218±0.027±0.016 where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model expectation

Study of W-boson polarisations and triple gauge boson couplings in the reaction e+e-→W+W- at LEP 2

“…The study of the longitudinal crosssection is particularly interesting as this degree of freedom of the W only arises in the Standard Model through the electroweak symmetry breaking mechanism. Measurements of the W polarisations at LEP have been reported previously by OPAL [7] and L3 [8]. The imaginary parts of the off-diagonal W + and W − SDM elements should vanish in the Standard Model and are particularly sensitive to CP-violation [9].…”

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

“…The direct measurement of the longitudinal cross-section for the single (i.e. uncorrelated) W production [1][2][3] is an important test of the same cancellation because the cause of the potential divergence of the cross-section is in the longitudinal parts. The measurement of the joint W W helicity states presented in this paper is an advance towards a more complete test of the Standard Model in the context of the W W reaction.…”

“…The smooth curve is from an analytic calculation using CC03 diagrams background, leaving events which are attributable to the CC03 signal. 3 The residue of the non-W W four-fermion background in the real data at all run energies is estimated to be at the level of 3 ± 2%, the uncertainty is due to combining results from different energies.…”

“…In the study of the reaction e − e + →W − W + at LEP2, the DELPHI Collaboration [1] and other LEP experiments [2,3] have established that on average ∼25% of W particles are longitudinally polarised, as predicted by the Standard Model. The present study measures the joint two-particle spin density matrix elements which give the probabilities that both W s are transversely polarised (W T W T ), both are longitudinally polarised (W L W L ) or that one W is transversely polarised while the associated W is longitudinal (W T W L + W L W T ).…”

“…All nine elements of the single-W particle spin density matrix can be determined using the data from semi-leptonic events. This was done in [1] and [2,3], where only the electron and muon decays of one W were used as the analyser of the W polarisation. The hadronic decays of W particles were not used because of the reasons outlined above and because the analysing power of polarisation in such decays is greatly reduced as the result of the severe practical difficulty to distinguish quark jets from anti-quark jets.…”