Measurement of the W-boson helicity in top-quark decays from t(t)over-bar production in lepton plus jets events in pp collisions at root s=7 TeV

The W-boson helicity fractions in top-quark decays are measured with tt events in the lepton+jets final state, using proton-proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 5.0 fb^(−1). The measured fractions of longitudinal, left-, and right-handed helicity are F_0 = 0.682 ± 0.030 (stat.) ± 0.033 (syst.), F_L = 0.310 ± 0.022 (stat.) ± 0.022 (syst.), and F_R = 0.008 ± 0.012 (stat.) ± 0.014 (syst.), consistent with the standard model predictions. The measured fractions are used to probe the existence of anomalous Wtb couplings. Exclusion limits on the real components of the anomalous couplings g_L , g_R are also derived.