Measurement of the W boson polarisation in $t\overline{t}$ events from pp collisions at $\sqrt{s}$ = 8 TeV in the lepton + jets channel with ATLAS

This paper presents a measurement of the polarisation of W bosons from $t\overline{t}$ decays, reconstructed in events with one high- ${p}_{\phantom{\rule{0.333333em}{0ex}}\text{T}\phantom{\rule{0.333333em}{0ex}}}$ lepton and at least four jets. Data from pp collisions at the LHC were collected at $\sqrt{s}$ = 8 TeV and correspond to an integrated luminosity of 20.2 fb ${}^{-1}$ . The angle ${\theta }^{\ast }$ 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 $cos{\theta }^{\ast }$ 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 ${F}_{0}=\phantom{\rule{3.33333pt}{0ex}}0.709\phantom{\rule{3.33333pt}{0ex}}±\phantom{\rule{3.33333pt}{0ex}}0.019$ , ${F}_{L}=\phantom{\rule{3.33333pt}{0ex}}0.299\phantom{\rule{3.33333pt}{0ex}}±\phantom{\rule{3.33333pt}{0ex}}0.015$ and ${F}_{R}=\phantom{\rule{3.33333pt}{0ex}}-0.008\phantom{\rule{3.33333pt}{0ex}}±\phantom{\rule{3.33333pt}{0ex}}0.014$ , and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.