Measurements of single top-quark production with the ATLAS detector

We present the measurement of the single top-quark $t$-channel production and evidence for the associated production of a $W$ boson together with a top-quark in proton-proton collisions at $\sqrt{s} = 7\,\mathrm{TeV}$ recorded with the ATLAS detector at the Large Hadron Collider. The $t$-channel production cross-section is measured using $1.04\,\mathrm{fb}^{-1}$ of collision data. The cross-section is measured by fitting the distribution of a neural network, yielding $\sigma_{t} = 83 \pm 4\mathrm{(stat.)} {}^{+20}_{-19}\mathrm{(syst.)} \,\mathrm{pb}$ in good agreement with the prediction of the Standard Model. Using the measured cross-section to extract $|V_{tb}|$ and assuming $|V_{tb}|\gg |V_{td}|,|V_{ts}|$ a result of $|V_{tb}|=1.13^{+0.14}_{-0.13}$ is obtained. Under the assumption of $0 < |V_{tb}| < 1$ a lower limit of $|V_{tb}|>0.75$ is set at the 95\% confidence level. We also present the measurement of the ratio of the top-quark and antitop-quark production in the $t$-channel $R_{t}= 1.81\pm 0.10\, (\mathrm{stat.})\, ^{+0.21}_{-0.20}\, (\mathrm{syst.})$ using $4.7\,\mathrm{fb^{-1}}$. %%WT We present evidence for the $Wt$ production in the dileptonic final state analysing $2.01\,\mathrm{fb}^{-1}$ of collision data. The $Wt$ production cross-section is determined by fitting the distribution of a multivariate discriminant constructed with a BDT, yielding $\sigma_{Wt} = 16.8 \pm 2.9 \mathrm{(stat.)} \pm 4.9 \mathrm{(syst.)}\,\mathrm{pb}$ in good agreement with the Standard Model expectation. The observed sensitivity of the measurement is $3.4\sigma$. From the cross-section measurement the CKM matrix element $|V_{tb}|=1.03^{+0.16}_{-0.19}$ is derived under the assumption that $|V_{tb}|\gg |V_{td}|,|V_{ts}|$.