Study of lepton flavor universality using top quark decay

Alsolami, Zainab and Borissov, Guennadi (2024) Study of lepton flavor universality using top quark decay. PhD thesis, Lancaster University.

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Abstract

Lepton flavor universality is one of the most important assumptions in the Standard Model, which states that all leptons (electrons, muons, taus) interact with the W boson with the same strength. This assumption can be tested by measuring the ratio of branching fractions of the decays (W → µν) and (W → eν), denoted by R(µ/e). To measure this ratio, the efficiency of the muon and electron is determined by using the decay of Z0 boson to muon and electron. Comparing the number of detected electrons and muons from Z boson decays reveals any efficiency differences between the detector’s response to these particles. The analysis is based on data collected by the ATLAS experiment during Run 2 of the Large Hadron Collider (LHC), from 2015 to 2018. This data corresponds to an integrated luminosity of 140 f b−1. The obtained uncertainty of R(µ/e) is 0.0064. In addition to the lepton flavor universality test, our study also investigates the beam position and luminosity measurement for the ATLAS detector during Run 3 of the LHC. This investigation focuses on changes in the beam spot position and partial widths across four runs taken in 2022: the van der Meer (vdM) run, two LHCf runs, and a physics run. A comparison of X, Y, and Z beam spot positions for the investigated runs reveals a consistent shape and direction for the scans in the vdM run and the other runs, with differences due to a crossing angle present in LHCf and physics runs. The symmetry of the scans in terms of the beam spot width was investigated by studying the dependence of the absolute value of the nominal separation. This revealed no asymmetry between the ascending and descending parts of the beam scans.