This paper presents a measurement of the polarisation of W bosons from [Formula: see text] decays, reconstructed in events with one high-[Formula: see text] lepton and at least four jets. Data from pp collisions at the LHC were collected at [Formula: see text] = 8 TeV and correspond to an integrated luminosity of 20.2 fb[Formula: see text]. The angle [Formula: see text] 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 [Formula: see text] 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 [Formula: see text], [Formula: see text] and [Formula: see text], and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.
Aaboud, M., Aad, G., Abbott, B., Abdallah, J., Abdinov, O., Abeloos, B., et al. (2017). Measurement of the W boson polarisation in tt¯ events from pp collisions at √s = 8 TeV in the lepton + jets channel with ATLAS. THE EUROPEAN PHYSICAL JOURNAL. C, PARTICLES AND FIELDS, 77(4), 264 [10.1140/epjc/s10052-017-4819-4].