Measurement of s-channel single top-quark production with the ATLAS detector using total event likelihoods

A measurement of s-channel single top-quark production in proton-proton collisions at a centre-of-mass energy of $8\,\mathrm{TeV}$ is presented. The dataset has been recorded with the ATLAS detector at the LHC and corresponds to an integrated luminosity of 20.3$\,\mathrm{fb}^{−1}$. Collision events are selected so that a subset of the data is obtained where the signal fraction is relatively high. Selected events contain one isolated electron or muon, missing transverse momentum and 2 jets, both of which are induced by b-quarks. All of these objects have large transverse momenta. The resulting set of events is still dominated by background processes, most notably top-quark pair production and the production of W-bosons in association with jets. In order to further separate the signal from the backgrounds, several approximate event likelihoods are computed. They are based on different hypotheses regarding the scattering process at hand. Together they result in a function of the measured momenta which allows for the desired separation of the signal process. A statistical model of the corresponding distribution is used in a fit to the measured data. The fit results in a signal significance of 3.4 standard deviations and a total cross section of $σ^{\mathrm{LHC,\,8\,TeV}}_{\mathrm{s−channel}} = 5.3^{+1.8}_{−1.6}\,\mathrm{pb}$ . This is the first evidence for s-channel single top-quark production in proton-proton collisions. The results agree with the standard model prediction.