  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.
