Measurement of the W boson polarisation in tt¯ events from pp collisions at s√ = 8 TeV in the lepton + jets channel with ATLAS

Abstract

Aquest article té una correcció a 10.1140/epjc/s10052-018-6520-7. ; Altres ajuts: We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of MINECO, Spain and EPLANET, ERC, ERDF, FP7, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union and CERCA Programme Generalitat de Catalunya. Funded by SCOAP3. ; This paper presents a measurement of the polarisation of W bosons from decays, reconstructed in events with one high- lepton and at least four jets. Data from pp collisions at the LHC were collected at = 8 TeV and correspond to an integrated luminosity of 20.2 fb. The angle 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 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 , and , and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.