"It is a well-known fact that the tWb vertex is primordial in order to perform high precision tests of the SM predictions in the top-quark sector. This vertex mediates both top-quark decays and single top-quark production. In the t-channel, which is the most dominant source of single top-quark production at the LHC, top-quarks are produced along with a so-called spectator light quark. According to SM predictions, these top-quarks will be highly polarized in the spectator quark direction given the vector-axial-vector form of the tWb vertex [1,2]. Furthermore, given that the top-quark decays almost exclusively to a W boson and a bottom quark, there are not QCD interactions to randomize its spin, allowing to access its spin orientation through the angular distribution of its decay products.
In the context of an effective field theory (EFT) the new physics corrections in the tWb vertex can be parametrized in terms of four couplings: VL,R and gL,R. In the SM at tree level the coupling VL is the Vtbelement of the CKM matrix that is close to one, while the anomalous couplings VR and gL,R are all zero. Deviations from these values would imply the presence of new physics and, concretely, complex phases in these parameters would imply that the top-quark has a CP-violating component.
These anomalous couplings would affect directly the angular distributions of the top-quark decay products, and therefore by measuring the distributions of three angles defined in Fig. 1, these parameters can be constrained.
In this project the dataset collected by the ATLAS experiment at 13 TeV center of mass energy during 2015, 2016, 2017 and 2018, corresponding to an integrated luminosity of 139.0 fb-1, is used to perform high precision measurements of the top-quark polarization in three directions"
 Gregory Mahlon and Stephen J. Parke. Single top quark production at the LHC: Understanding spin. Phys. Lett., B476:323–330, 2000.
 Reinhard Schwienhorst, C. P. Yuan, Charles Mueller, and Qing-Hong Cao. Single top quark production and decay in the t-channel at next-to-leading order at the LHC. Phys. Rev., D83:034019, 2011.