Evidence for s-channel single top-quark production in $pp$ collisions at $\sqrt{s}=8\,\text{TeV}$ with the ATLAS detector

Evidence for single top-quark production in the $s$-channel using proton--proton collisions at a centre-of-mass energy of $8\,\mathrm{TeV}$ with the \ATLAS detector at the \CERN Large Hadron Collider. The analysis is performed on events containing one isolated electron or muon, large missing transverse momentum and exactly two $b$-tagged jets in the final state. The analysed data set corresponds to an integrated luminosity of $20.3\mathrm{fb}^{-1}$. The signal is extracted using a maximum-likelihood fit of a discriminant which is based on the matrix element method and optimized in order to separate single-top-quark $s$-channel events from the main background contributions, which are top-quark pair production and $W$ boson production in association with heavy-flavour jets. The measurement leads to an observed signal significance of 3.2 standard deviations and a measured cross-section of $\sigma_s\!=\!4.8\!\pm\!0.8\text{(stat.)}^{+1.6}_{-1.3}\text{(syst.)}\mathrm{pb}$, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.