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

A first measurement of the top quark spin asymmetry, sensitive to the top quark polarisation, in $t$-channel single top quark production is presented. It is based on a sample of pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 $\mathrm{fb^{-1}}$. A high-purity sample of $t$-channel single top quark events with an isolated muon is selected. Signal and background components are estimated using a fit to data. A differential cross section measurement, corrected for detector effects, of an angular observable sensitive to the top quark polarisation is performed. The differential distribution is used to extract a top quark spin asymmetry of $0.26 \pm 0.03 \textrm{(stat)} \pm 0.10 \textrm{(syst)}$, which is compatible with a p-value of $4.6\%$ with the standard model prediction of 0.44.