Tau lepton identification and universality tests with the ATLAS experiment

Abstract

This thesis presents a new method to measure tau lepton universality in W boson decays at the LHC. Ratios of lτ had and eμ final states are compared in tt̄ and Z → τ + τ − decays, with l representing either a muon or electron, and τ had is a tau lepton which decays hadronically. The double-ratio cancels uncertainties from τ had lepton identification, which can be large at hadron colliders, and enables a precision test of tau lepton universality at the level of ∼ 1.5%. This method is applied to data collected by the ATLAS experiment during 2015–2018 data taking at s = 13 TeV, and indicates that lepton universality can be tested with ATLAS data at a precision of 1.6%. However there still remain differences between the expected Z → τ + τ − yields of the nominal and systematic signal simulation samples. These differences need to be understood before a precise measurement of lepton universality can be performed. In addition, a calibration of the τ had lepton identification methods used by the ATLAS experiment during 2015–2018 data taking at s = 13 TeV is presented. The efficiency of the τ had identification is measured in both data and simulation, and the resulting differences between the efficiency measurements are used to correct the efficiency in simulation to the observations in data.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Terence Wyatt (Supervisor) & Alexander Oh (Supervisor)