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

The European Physical Journal C, Apr 2017

This paper presents a measurement of the polarisation of W bosons from \(t\bar{t}\) decays, reconstructed in events with one high-\(p_{\text{ T }}\) 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.

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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

Eur. Phys. J. C Measurement of the W boson polarisation in t t¯ events from p p collisions at √s = 8 TeV in the lepton + jets channel with ATLAS ATLAS Collaboration 0 0 CERN , 1211 Geneva 23 , Switzerland This paper presents a measurement of the polarisation of W bosons from t t¯ decays, reconstructed in events with one high- pT 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 cos θ ∗ 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 W t b vertex are set. - The top quark, discovered in 1995 by the CDF and D0 collaborations [1,2] is the heaviest known elementary particle. It decays almost exclusively into a W boson and a b-quark. The properties of the top decay vertex W t b are determined by the structure of the weak interaction. In the Standard Model (SM) this interaction has a (V − A) structure, where V and A refer to the vector and axial vector components of the weak coupling. The W boson, which is produced as a real particle in the decay of top quarks, possesses a polarisation which can be left-handed, right-handed or longitudinal. The corresponding fractions, referred to as helicity fractions, are determined by the W t b vertex structure and the masses of the particles involved. Calculations at nextto-next-to-leading order (NNLO) in QCD predict the fractions to be FL = 0.311 ± 0.005, FR = 0.0017 ± 0.0001, F0 = 0.687 ± 0.005 [3]. By measuring the polarisation of the W boson with high precision, the SM prediction can be tested, and new physics processes which modify the structure of the W t b vertex can be probed. The structure of the W t b vertex can be expressed in a general form using left- and right-handed vector (VL/R) and tensor (gL/R) couplings: Here, PL/R refer to the left- and right-handed chirality projection operators, mW to the W boson mass, and g to the weak coupling constant. At tree level, all of the vector and tensor couplings vanish in the SM, except VL, which corresponds to the CKM matrix element Vtb and has a value of approximately one. Dimension-six operators, introduced in effective field theories, can lead to anomalous couplings, represented by non-vanishing values of VR, gL and gR [4–6]. The W boson helicity fractions can be accessed via angular distributions of polarisation analysers. Such analysers are W boson decay products whose angular distribution is sensitive to the W polarisation and determined by the W t b vertex structure. In case of a leptonic decay of the W boson (W → ν), the charged lepton serves as an ideal analyser: its reconstruction efficiency is very high and the sensitivity of its angular distribution to the W boson polarisation is maximal due to its weak isospin component T3 = − 21 . If the W boson decays hadronically (W → qq¯ ), the downtype quark is used, as it carries the same weak isospin as the charged lepton. This provides it with the same analysing power as the charged lepton, which is only degraded by the lower reconstruction efficiency and resolution of jets compared to charged leptons. The reconstruction of the downtype quark is in particular difficult as the two decay products of a hadronically decaying W boson are experimentally hard to separate. In the W boson rest frame, the differential crosssection of the analyser follows the distribution which directly relates the W boson helicity fractions Fi to the angle θ ∗ between the analyser and the reversed direction of flight of the b-quark from the top quark decay in the W boson rest frame. Previous measurements of the W boson helicity fractions from the ATLAS, CDF, CMS and D0 collaborations show agreement with the SM within the uncertainties [7–11]. In this paper, the W boson helicity fractions are measured in top quark pair (t t¯) events. Data corresponding to an integrated luminosity of 20.2 fb−1of proton–proton ( pp) collisions, produced at the LHC with a centre-of-mass energy of √s = 8 TeV, and recorded with the ATLAS [12] detector, are analysed. The final state of the t t¯ events is characterised by the decay of the W bosons. This analysis considers the lepton+jets channel in which one of the W bosons decays leptonically and the other decays hadronically. (...truncated)


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Calvente Lopez, D. Calvet. 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, The European Physical Journal C, 2017, pp. 264, Volume 77, Issue 4, DOI: 10.1140/epjc/s10052-017-4819-4