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 is made using 255 ${\text{pb}}^{-1}$ of proton-proton collision data recorded at $\sqrt{s}=5.02$ TeV with the ATLAS detector at the Large Hadron Collider. An event selection is used to identify single-top-quark candidates arising from $t$-channel production with the top quark decaying semi-leptonically. Events passing the selection are then 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 $R_t$ between these two. They are measured to be $\sigma (tq+\bar{t}q)=27.1_{-4.1}^{+4.4}(\text{stat.}{)}_{-3.7}^{+4.4}(\text{syst.})$ pb and $R_t=2.73_{-0.82}^{+1.43}(\text{stat.}{)}_{-0.29}^{+1.01}(\text{syst.})$. The individual single-top-quark ($tq$) and single-top-antiquark ($\bar{t}q$) production cross-sections are measured to be $\sigma (tq)=19.8_{-3.1}^{+3.9}(\text{stat.}{)}_{-2.2}^{+2.9}(\text{syst.})$ pb and $\sigma (\bar{t}q)=7.3_{-2.1}^{+3.2}(\text{stat.}{)}_{-1.5}^{+2.8}(\text{syst.})$ pb. All measurements are in good agreement with the Standard Model predictions.