Observation of $t\bar{t}$ production in lepton+jets and dilepton channels in $p$+Pb collisions at $\sqrt{s_\mathrm{NN}}=8.16\;\mathrm{TeV}$ with the ATLAS detector

This note presents a measurement of the top-quark pair production cross section in proton-lead~($p$+Pb) collisions at $\sqrt{s_\mathrm{NN}}=8.16$ TeV at the Large Hadron Collider. The dataset was recorded with the ATLAS detector in 2016 and amounts to an integrated luminosity of 165 nb$^{-1}$. Top-quark pair production is observed in the lepton+jets and the dilepton channels, with a significance well over 5 standard deviations in both channels separately. Combining both channels, the top-quark pair production cross section is measured to be $$\sigma_{t\bar{t}}= 57.9\pm 2.0\;\mathrm{(stat.)\;^{+4.9}_{-4.5} ;\mathrm{(syst.)}}\;\mathrm{nb},$$ with a total relative uncertainty of 9\%. The result is compared with theory predictions involving various nuclear parton distribution functions as well as to the same quantity measured in proton-proton collisions extrapolated from the proton-proton system at 8 TeV. A good agreement within total uncertainties is found.