Evidence for single top-quark production in the s-channel in proton–proton collisions at s = 8 ? TeV with the ATLAS detector using the Matrix Element Method

摘要

This 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 σ s = 4.8 ± 0.8 (stat.) ? 1.3 + 1.6 (syst.) ?pb , which is consistent with the Standard Model expectation. The expected significance for the analysis is 3.9 standard deviations.