Observation of top-quark pair production in heavy-ion collisions in the ATLAS experiment at the LHC

Hard probes are expected to provide crucial input for nuclear parton distribution functions (nPDF), as well as to bring valuable insights into the quark-gluon plasma (QGP). In this thesis, measurements of heavy-ion collisions using top-quark pairs ($t\bar{t}$) in the ATLAS experiment at the Large Hadron Collider are presented. Analysed data from proton-lead ($p$+Pb) and lead-lead (Pb+Pb) collisions were collected with the ATLAS detector during Run 2 (2015-2018) at a nucleon-nucleon centre-of-mass energy of 8.16 and 5.02 TeV, respectively. The conducted studies open a new path for further research on heavy-ion collisions at ultra-relativistic energies. The precise measurement of $t\bar{t}$ production in $p$+Pb collisions provides valuable input for constraining nPDFs in the high Bjorken-$x$ region. The observation of the $t\bar{t}$ process in Pb+Pb collisions marks the start of the heavy-ion program with top quarks, and in particular, opens a possibility of exploring the time structure of the QGP in the future.