  This Letter presents a search at the LHC for s-channel single top-quark
production in proton-proton collisions at a centre-of-mass energy of 8 TeV. The
analyzed data set was recorded by the ATLAS detector and corresponds to an
integrated luminosity of 20.3 fb$^{-1}$. Selected events contain one charged
lepton, large missing transverse momentum and exactly two b-tagged jets. A
multivariate event classifier based on boosted decision trees is developed to
discriminate s-channel single top-quark events from the main background
contributions. The signal extraction is based on a binned maximum-likelihood
fit of the output classifier distribution. The analysis leads to an upper limit
on the s-channel single top-quark production cross-section of 14.6 pb at the
95% confidence level. The fit gives a cross-section of $\sigma_s$=5.0$\pm$4.3
pb, consistent with the Standard Model expectation.
