Search for Anomalous Coupling in Wtb Vertex from the Measurement of Triple Differential Angular Decay Rates of Single Top Quarks Produced in the T-channel at √s =8 Tev with ATLAS Detector
The electroweak production and subsequent decay of single top quarks is determined by
the properties of the Wtb vertex. This vertex can be described by the complex parameters of an effective Lagrangian. An analysis of angular distributions of the decay products of single top quarks produced in the t-channel constrains these parameters simultaneously. The thesis presents this analysis which uses 20.2 fb−1 of proton–proton collision data at a centre-of-mass energy of 8 TeV collected with the ATLAS detector at the LHC. It uses the triple-differential decay rate in t-channel production to determine six observables simultaneously, five generalised helicity fractions and phases, as well as the polarisation of the produced top quark. Detector effects are deconvolved from the measured differential rates using Fourier techniques, in particular a variant of the ordinary convolution theorem. The fraction f1 of decays containing transversely polarised W bosons is measured to be f = 0.296+0.048 −0.051 (stat. + syst.). The phase δ_ between amplitudes for transversely and longitudinally polarised W bosons recoiling against left-handed b quarks, is measured to be δ_ = 0.002π+0.016π−0.017π (stat. + syst.), giving no indication of CP violation. The fraction of longitudinal to transverse W bosons accompanied by right-handed b-quarks are also constrained at 95% C.L. to f1+ < 0.118 and f0+ < 0.085. Based on these measurements limits are placed at 95% C.L. on the ratio of the complex coupling parameters gR and VL such that Re [gR /VL ] ∈ [−0.122, 0.168] and Im [gR /VL ] ∈ [−0.066, 0.059]. Constraints are also placed on the magnitudes of the ratios |VR/VL|, and |gL/VL|. Finally the polarisation of single top quarks in the t-channel is constrained to be P > 0.718 (95% C.L.). None of the above measurements make assumptions on the value of any of the other parameters or couplings and all of them are in agreement with the Standard Model.