Measurement of the inclusive cross-section of single top-quark $t$-channel production in $pp$ collisions at $\sqrt{s}$ = 13 TeV

A measurement of the $t$-channel single top-quark production cross-section in the lepton+jets channel using 3.2 fb$^{-1}$ $pp$ collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector in 2015, is presented. Events are selected by requiring one muon, missing transverse momentum, and two high-$p_{\mathrm T}$ jets, one of which is required to be $b$-tagged. Using a binned maximum-likelihood fit to the output distribution of a neural network, the cross-sections are determined to be $\sigma(tq) = 133 \pm 6 \, (\mathrm{stat.}) \pm 24 \, (\mathrm{syst.}) \pm 7 \,(\mathrm{lumi.})~\text{pb}$ for single top-quark production and $\sigma(\bar{t} q) = 96 \pm 5 \, (\mathrm{stat.}) \pm 23 \, (\mathrm{syst.}) \pm 5\,(\mathrm{lumi.})~\text{pb}$ for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. Using the predicted value of $\sigma(tq)$ at next-to-leading order 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 $Wtb$ vertex is determined to be $|f_{\text{LV}} \cdot V_{tb}|=1.03 \pm 0.11$. If it is assumed that $|V_{tb}|\leq 1$ and $f_{\text{LV}}=1$, a lower limit of $|V_{tb}|> 0.78$ is obtained at the 95 \% confidence level.