The European Physical Journal C > 2017 > 77 > 4 > 1-28
Measurement of the W boson polarisation in $$t\bar{t}$$ tt¯ events from pp collisions at $$\sqrt{s}$$ s = 8 TeV in the lepton + jets channel with ATLAS
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Abstract
This paper presents a measurement of the polarisation of W bosons from $$t\bar{t}$$ tt¯ decays, reconstructed in events with one high-$$p_{\text{ T }}$$ pT lepton and at least four jets. Data from pp collisions at the LHC were collected at $$\sqrt{s}$$ s = 8 TeV and correspond to an integrated luminosity of 20.2 fb$$^{-1}$$ -1 . The angle $$\theta ^{*}$$ 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 $$\cos {\theta ^{*}}$$ 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 $$F_{\mathrm {0}}=~0.709~\pm ~{0.019}$$ F0=0.709±0.019 , $$F_{\mathrm {L}}=~0.299~\pm ~{0.015}$$ FL=0.299±0.015 and $$F_{\mathrm {R}}=~-0.008~\pm ~{0.014}$$ 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.
Identifiers
journal ISSN : | 1434-6044 |
journal e-ISSN : | 1434-6052 |
DOI | 10.1140/epjc/s10052-017-4819-4 |