Measurement of single top-quark production in the s-channel in proton-proton collisions at $\mathrm{\sqrt{s}=13~TeV}$ with the ATLAS detector

A measurement of single top-quark production in the s-channel is performed in proton-proton collisions at a centre-of-mass energy of $\mathrm{13~TeV}$ with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of $\mathrm{139~fb^{-1}}$. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two $b$-tagged jets in the final state. A discriminant based on matrix element calculations is used 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 jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is $\sigma=\mathrm{8.2^{+3.5}_{-2.9}~pb}$, consistent with the Standard Model prediction of $\sigma^{\mathrm{SM}}=\mathrm{10.32^{+0.40}_{-0.36}~pb}$.