Measurement of lepton efficiency in lead-lead collisions in the ATLAS experiment at the LHC

Lepton (electron and muon) efficiencies are calculated in lead-lead (Pb+Pb) collisions at a center-of-mass energy of 5.02 TeV per nucleon pair, with an integrated luminosity of 1.9 nb-1. The data collected during Run 2 by the ATLAS experiment, utilizing a multi-purpose detector operating at the Large Hadron Collider (LHC), are analyzed. The tag-and-probe method is implemented, focusing on the Z boson, which decays into same-flavor opposite-charge lepton pairs. The analysis of data and Monte Carlo (MC) simulation leads to the determination of efficiencies as a function of kinematic variables and final outcomes, i.e., scale factors (SFs), along with statistical and systematic uncertainties, at three stages of lepton recognition: identification, isolation, and trigger. These SFs are used to correct for mismodelling of MC simulation in the first measurement of top-quark pair production in Pb+Pb collisions performed by the ATLAS Collaboration [1]. The ATLAS TagAndProbe software is utilized in the analysis of the data collected in 2024 heavy-ion (HI) run, part of the Run 3 at the LHC, at the center-of-mass energy of 5.36 TeV. The Z boson decaying into electron pairs is used in the evaluation of electron performance in proton-proton (pp) reference collisions and Pb+Pb collisions, with an integrated luminosity of 425 pb-1 and 1.67 nb-1, respectively. Control distributions of the invariant mass of the tag-and-probe pairs and key kinematic variables of probes are analyzed for various requirements on their working points (WPs) for identification, using fixed WPs for tags: Tight for pp collisions and HIMedium for Pb+Pb collisions. The analysis revealed subtle imperfections in the detection geometry and demonstrated excellent data quality from Pb+Pb collisions, including up to 75,035 electron pairs from Z-boson decay events for future studies. The thesis was developed in close collaboration with Patrycja Potępa. References: [1] e-Print 2411.10186 [hep-ex], CERN-EP-2024-282 (2024)