Measurement of $t$-Channel Single-Top-Quark Production in $pp$ Collisions at $\sqrt{s} = 5.02$ TeV with the ATLAS Detector

The observation of the electroweak production of single-top-quarks using 257 $\text{pb}^{-1}$ of proton-proton collision data recorded at $\sqrt{s} = 5.02$ TeV with the ATLAS detector at the Large Hadron Collider is reported. An event selection optimised for $t$-channel production with semileptonic decays of the top quark is used to measure the inclusive cross-section for the combined production of single-top-quarks and antiquarks ($\sigma({tq + \bar{t}q})$) and the ratio between these two ($R_t$). The combined production cross-section and ratio are measured to be $\sigma({tq + \bar{t}q}) = 26.6_{-4.0}^{+4.3}~(\text{stat})~_{-3.6}^{+4.4}~(\text{syst})$ pb and $R_t = 2.74_{-0.83}^{+1.44}~(\text{stat})~_{-0.29}^{+1.04}~(\text{syst})$. The cross-section measurement corresponds to a significance of 6.1 standard deviations for $t$-channel single-top-quark production with respect to the background-only hypothesis. The individual single-top-quark ($t q$) and single-top-antiquark ($\bar{t} q$) production cross-sections are measured to be $\sigma(t q) = 19.5_{-3.1}^{+3.8}~(\text{stat})~_{-2.2}^{+2.9}~(\text{syst})$ pb and $\sigma(\bar{t}q) = 7.1_{-2.1}^{+3.2}~(\text{stat})~_{-1.5}^{+2.8}~(\text{syst})$ pb. All measurements are in good agreement with the Standard Model predictions.