Measurement of muon trigger efficiency using the Tag and Probe method in ultra-peripheral heavy-ion collisions of lead-lead at 5.36 TeV in the ATLAS experiment at the LHC

This thesis presents a study of the muon Level-1 trigger efficiency in lead-lead (Pb-Pb) heavy-ion collisions recorded by the ATLAS experiment at the centre-of-mass energy equal to 5.36 TeV in 2023 (Run 3) at the Large Hadron Collider (LHC). The analysis focuses on ultra-peripheral collisions (UPC), in which both nuclei pass each other at large impact parameter, greater than twice the nuclear radius of Pb. The UPCs are dominated by the electromagnetic processes, where the hadronic interactions are strongly suppressed.

The measured results of the trigger efficiency are compared with Monte Carlo (MC) simulation obtained from the SuperChic MC generator interfaced with Pythia8. The signal selection is applied to isolate the two-photon muon-pair production process in both data and MC. For the selected data and MC samples the tag-and-probe method is employed to measure the trigger efficiencies. The results of the trigger efficiencies are showed as the transverse momentum versus charge-signed psuedorapidity efficiency maps and as one-dimensional projections versus transverse momentum in 11 charge-signed pseudorapidity intervals. The scale factors (SFs) used in the physics analyses to correct the MC simulation are derived as the ratio of the efficiencies estimated in data to those from MC simulation.

The statistical uncertainties of the trigger efficiency are calculated using the Clopper-Pearson method. This method yields asymmetric confidence interval for binomial proportions. The statistical uncertainties of the SFs are then obtained by propagating the upper and lower uncertainties separately. Moreover, the systematic uncertainties are estimated from several sources and compared to the statistical uncertainties to determine which one dominates.

Finally, the preliminary analysis of the 2025 Pb+Pb data collected at the centre-of-mass energy equal to 5.36 TeV is performed, together with the comparison of the reference 2023 data. The 2025 data used similar selection, and the Level-1 trigger efficiencies are measured using the same tag-and-probe method.