Direct Observation of Longitudinally Polarised W Bosons

TL;DR

This study directly observes the polarization states of W bosons produced in electron-positron collisions, confirming the Standard Model prediction of their longitudinal polarization fraction.

Contribution

It provides the first direct measurement of all three helicity states of W bosons in e+e- collisions at these energies.

Findings

W bosons are produced with all three helicities.

Measured longitudinal polarization fraction is 0.261 +/- 0.051(stat.) +/- 0.016(syst.).

Results agree with Standard Model predictions.

Abstract

The three different helicity states of W bosons, produced in the reaction e+e- -> W+W- -> l nu q q~ are studied using leptonic and hadronic W decays at sqrt{s}=183GeV and 189GeV. The W polarisation is also measured as a function of the scattering angle between the W- and the direction of the e- beam. The analysis demonstrates that W bosons are produced with all three helicities, the longitudinal and the two transverse states. Combining the results from the two center-of-mass energies and with leptonic and hadronic W decays, the fraction of longitudinally polarised W bosons is measured to be 0.261 +/- 0.051(stat.) +/- 0.016(syst.) in agreement with the expectation from the Standard Model.