Measurement of the Inclusive and Fiducial Cross-Section of Single Top-Quark $t$-Channel Events in $pp$ Collisions at $\sqrt{s}$ = 8 TeV

This note presents the measurement of a $t$-channel single top-quark production fiducial cross-section in the lepton+jets channel with 20.3 fb$^{-1}$ of 8 TeV data using a neural-network discriminant. Events are selected by requiring exactly two jets, where one of the jets is required to be $b$-tagged. Signal events from $t$-channel single top-quark processes are enhanced using a neural-network discriminant based on final-state observables. The $t$-channel production cross-section in the fiducial region is obtained from a binned maximum-likelihood fit to the neural-network discriminant. A fiducial cross-section quoted within the detector acceptance of $\sigma_{\rm fid} = 3.37 \pm 0.05 \, (\mathrm{stat.}) \pm 0.47 \, (\mathrm{syst.}) \pm 0.09 \, (\mathrm{lumi.})~\text{pb}$ is obtained. The total inclusive $t$-channel cross-section is calculated using the acceptance predicted by various Monte Carlo generators. If the acceptance from the aMC@NLO+Herwig event generator is used, a value of $\sigma_t = 82.6 \pm 1.2 \, (\mathrm{stat.}) \pm 11.4 \, (\mathrm{syst.}) \pm 3.1 \, (\mathrm{PDF}) \pm 2.3 \, (\mathrm{lumi.})~\text{pb}$ is obtained, consistent with the Standard Model prediction. Using the ratio of the measured inclusive cross-section to the predicted cross-section and assuming that the top-quark-related CKM matrix elements obey the relation $|V_{tb}|\gg |V_{ts}|, |V_{td}|$, the coupling strength at the $W$-$t$-$b$ vertex is determined to be $|V_{tb}|=0.97^{+0.09}_{-0.10}$. Assuming that $|V_{tb}|\leq 1$ a lower limit of $|V_{tb}|>0.78$ is obtained at the 95% confidence level.