Measurement of top quark polarization in t-channel single-top production

This note documents the measurement of top-quark polarization in t-channel single-top production, analyzing approximately 20 fb$^{-1}$ of pp collisions at a center-of-mass energy of 8 TeV. A high-purity sample of t-channel single-top events is defined, signal and background components are estimated by a fit to data, and an unfolding technique is applied to infer the parton-level distribution of an angular observable sensitive to the top-quark polarization. The unfolded distribution, which is compatible with the standard model expectation, is used to extract the top-quark spin asymmetry, $A_l = 0.41 \pm 0.06 (stat.) \pm 0.16 (syst.)$. That corresponds to a top-quark polarization $P_t = 0.82 \pm 0.12 (stat.) \pm 0.32 (syst.)$ under the assumption that the spin analyzing power of a charged lepton is $100\%$.