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

Abstract : This paper presents a measurement of the polarisation of $W$ bosons from $t\bar{t}$ decays, reconstructed in events with one high-$p_{\mbox{T}}$ 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 $\theta^{*}$ 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 $\cos{\theta^{*}}$ 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 $F_{\mathrm{0}}= 0.709 \pm {0.019}$, $F_{\mathrm{L}}= 0.299 \pm {0.015}$ and $F_{\mathrm{R}}= -0.008 \pm {0.014}$, and are the most precisely measured $W$ boson polarisation fractions to date. Limits on anomalous couplings of the $Wtb$ vertex are set.
Type de document :
Article dans une revue
European Physical Journal C: Particles and Fields, Springer Verlag (Germany), 2017, 77, pp.264. 〈https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-017-4819-4〉. 〈10.1140/epjc/s10052-017-4819-4〉

http://hal.in2p3.fr/in2p3-01412923
Contributeur : Claudine Bombar <>
Soumis le : vendredi 9 décembre 2016 - 09:28:04
Dernière modification le : lundi 22 octobre 2018 - 09:42:01

### Citation

M Aaboud, N. Berger, A.M. Burger, M. Delmastro, L. Di Ciaccio, et al.. Measurement of the $W$ boson polarisation in $t\bar{t}$ events from $pp$ collisions at $\sqrt{s}$ = 8 TeV in the lepton+jets channel with ATLAS. European Physical Journal C: Particles and Fields, Springer Verlag (Germany), 2017, 77, pp.264. 〈https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-017-4819-4〉. 〈10.1140/epjc/s10052-017-4819-4〉. 〈in2p3-01412923〉

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