Probing the $Wtb$ vertex structure with $t$-channel single top-quark events in $pp$ collisions at $\sqrt{\mathrm{s}}=8$ TeV with the ATLAS detector

To probe the $Wtb$ vertex structure, top-quark and $W$ boson polarisation observables are measured from $t$-channel single top-quark events produced in proton--proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb$^{-1}$, recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a $b$-hadron. Stringent selection requirements are applied to discriminate $t$-channel single top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantization axes appropriately chosen for the top quark and the $W$ boson. The asymmetry measurements are performed by correcting the observed angular distributions for detector and physics effects after subtracting the background contributions. The measured top-quark and $W$ boson polarisation values are in good agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling $g_{\rm R}$ are also set from model-independent measurements