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

In: European Physical Journal C: Particles and Fields, Vol. 77, 264, 04.2017, p. 1-28.

Research output: Contribution to journalArticlepeer-review

  • The ATLAS Collaboration


This paper presents a measurement of the polarisation of W bosons from tt¯ decays, reconstructed in events with one high-p T lepton and at least four jets. Data from pp collisions at the LHC were collected at √s = 8 TeV and correspond to an integrated luminosity of 20.2 fb−1. The angle θ∗ 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 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, FL= 0.299 ± 0.015 and FR= −0.008 ± 0.014, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the Wtb vertex are set.
Original languageEnglish
Article number264
Pages (from-to)1-28
Number of pages28
JournalEuropean Physical Journal C: Particles and Fields
Early online date26 Apr 2017
Publication statusPublished - Apr 2017
This open access research output is licenced under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

ID: 28140594