Z boson production in 5.02 TeV proton-proton and lead-lead data measured by the ATLAS experiment at the LHC

The study of electroweak bosons in proton-proton, proton-nucleus and nucleus-nucleus collisions provide an important insight into initial state effects of hadron and nuclear collisions, such as collision geometry and modification of parton distribution functions. This research focuses on studying the $Z$ boson production in Run-2 data collected with the ATLAS detector at the Large Hadron Collider. Measurements are obtained with the data from proton-proton and nucleus-nucleus collisions at a center-of-mass energy of 5.02 TeV per nucleon pair. Fiducial $Z$-boson cross section in proton-proton data is measured in electron and muon decay channels. The combined cross section is found to be in good agreement with next-to-next-to-leading-order QCD calculations. Rapidity differential cross section is found to agree with calculations within 1 to 2 standard deviations, with the biggest discrepancy measured in the rapidity region $|y|<1$. The results from the $pp$ system are used to form the nuclear modification factor of the previously measured $Z$-boson cross section in $p$+Pb collisions. The nuclear modification factor, measured as a function of $Z$ boson rapidity, shows an asymmetry qualitatively consistent with nuclear modification of the parton distribution functions. This asymmetry is more pronounced in central $p$+Pb collisions compared to peripheral events. In the Pb+Pb system, the $Z$-boson differential cross section is measured in electron and muon decay channels as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with the measured proton-proton results. The nuclear modification factor is found to be consistent with unity in central and mid-central collisions. After rejecting photon-induced backgrounds, the nuclear modification factor in peripheral collisions is found to be above unity, although consistent with it. The results are compared with theoretical predictions obtained at next-to-leading-order using proton and nuclear parton distribution functions. The measured $Z$ boson yields in Pb+Pb collisions lie about 1 to 3 standard deviations above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity while showing a trend consistent with next-to-leading-order QCD calculation including the isospin effect.