Measurement of Electron Reconstruction Efficiencies in Run 2 and Run 3 at the ATLAS Experiment

To uncover evidence for new laws of nature, the Standard Model of particle physics is probed at high-energy colliders with increasing precision. The wide range of surveyed processes has to be reconstructed from only a few directly observable final-state objects. A precise understanding of the behavior of these objects in the detector, as well as the performance of the employed algorithms, is therefore crucial.

This thesis presents the measurement of the electron reconstruction efficiency in LHC Run 2 and Run 3 at the ATLAS experiment, corresponding to a total integrated luminosity of 307.1 inverse femtobarn. The efficiency is determined using the tag-and-probe method applied to 𝑍 → 𝑒𝑒 decays together with data-driven background estimation techniques. The comparison of efficiencies between data and simulation enables the derivation of scale factors, which are used by physics analyses involving electron final-states.

When integrated over all included kinematic regions, an overall reconstruction efficiency of 98 % is measured with permille-level uncertainties. While in Run 2 the scale factors are close to unity, several deviations between efficiencies in data and simulation are observed in Run 3, which is subsequently investigated. To further improve the stability of the results and reduce their uncertainties, major changes in background estimation fitting procedure are evaluated.