Abstract:
Observation of ttbar production in lepton+jets and dilepton channels in p+Pb collisions at √s = 8.16 TeV with the ATLAS detector. Top-quarks and Higgs boson are the only elementary particles that have not been observed in heavy-ion collisions in the ATLAS detector yet. In particular top quarks, the heaviest elementary particles carrying color charges, have been argued to be attractive candidates for probing the quark-gluon plasma produced in heavy-ion collisions. In proton-lead collisions, top-quark production is expected to be sensitive to nuclear modifications of parton distribution functions (PDF) at high Bjoerken-x values which are hard to access experimentally using other probes available so far. I participate in the analysis of measurement of the top quark pairs production in proton-lead collisions at √s=8.16 TeV in the ATLAS experiment. The dataset was recorded in 2016 and corresponds to an integrated luminosity of 165 nb-1. The top quarks are reconstructed in lepton+jet and di-lepton decay channels with electrons and muons in the final state. The dilepton channel is studied in heavy-ion collisions for the first time. The main task from my side was to estimate the background contribution to the signal sample. These results were important inputs to the analysis. Namely, the inclusive ross section measurement for the top-quark pairs production in dilepton and lepton+jets decay modes with electrons and muons recorded by the ATLAS experiment. The measurement is compared to the NNLO predictions for top-quark production using various PDF sets.