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

This note 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 analyzed 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 $\sigma_s\!=\!4.8\!\pm\!1.1$(stat.)$^{+2.2}_{-2.0}$(syst.)$\,$pb which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.