UC Irvine

This paper presents a measurement of the polarisation of W bosons from tt¯$$t\bar{t}$$ decays, reconstructed in events with one high-pT$$p_{\text{ T }}$$ lepton and at least four jets. Data from pp collisions at the LHC were collected at s$$\sqrt{s}$$ = 8 TeV and correspond to an integrated luminosity of 20.2 fb-1$$^{-1}$$. The angle θ∗$$\theta ^{*}$$ between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cosθ∗$$\cos {\theta ^{*}}$$ is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are F0=0.709±0.019$$F_{\mathrm {0}}=~0.709~\pm ~{0.019}$$, FL=0.299±0.015$$F_{\mathrm {L}}=~0.299~\pm ~{0.015}$$ and FR=-0.008±0.014$$F_{\mathrm {R}}=~-0.008~\pm ~{0.014}$$, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.