Evidence for single top-quark production in the $s$-channel in proton-proton collisions at $\sqrt{s}=$8 TeV with the ATLAS detector using the Matrix Element Method

his Letter presents evidence for single top-quark production in the?s-channel using proton-proton collisions at a centre-of-mass energy of 8 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 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?σs=4.8±0.8(stat.)+1.6?1.3(syst.) pb, which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.