Observation of top-quark pair production in proton-lead collisions in ATLAS

Abstract

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 colour 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. In 2016 the ATLAS experiment collected proton-lead collisions at centre-of-mass energy of 8.16 TeV per nucleon pair. The data sample corresponds to an integrated luminosity of 164 nb-1, which allows for the first time in this data set with ATLAS, to measure top-quark pair production. In this work, we discuss the inclusive cross 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.