A test of Lepton Flavour Universality by comparing the decay widths of W bosons produced in top quark pair events with the ATLAS detector at the LHC

Sanderswood, Izaac and Borissov, Guennadi (2021) A test of Lepton Flavour Universality by comparing the decay widths of W bosons produced in top quark pair events with the ATLAS detector at the LHC. PhD thesis, Lancaster University.

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Abstract

At this time, the Standard Model of particle physics represents our best understanding of physics at the smallest scales. A fundamental axiom of this theory is the universality of the couplings between the different lepton generations and the gauge bosons. This principle is known as Lepton Flavour Universality and can be tested by comparing the decay widths of (semi-)leptonic processes that differ only in lepton flavour. Such a comparison of the decay widths of W bosons to taus and muons can be quantified by R(τ/μ) = Br(W → τν)/Br(W → μν). A measurement of R(τ/μ) is presented in this thesis. The measurement uses W bosons produced in top pair decays with a dileptonic decay mode. The measurement is based on 139 fb−1 of pp collision data recorded with the ATLAS detector at the Large Hadron Collider (LHC) at √s = 13 TeV. In this analysis, muons originating from W bosons and those originating via an intermediate tau are distinguished by the lifetime of the tau, using the transverse impact parameter of the muons, as well as differences in the transverse momenta spectra of the muons. The observed best-fit value of R(τ/μ) is 0.992 ± 0.013[±0.007(stat) ± 0.011(syst)] and is therefore in agreement with the hypothesis of universal lepton couplings, as postulated in the Standard Model. This is the most precise measurement of R(τ/μ), surpassing the previous LEP measurement by a factor 2. This result is consistent with the LEP measurement at the level of 2.7 standard deviations. Such results would not be possible without the continued performance of the ATLAS detector. Primary vertices represent the locations of proton-proton collisions – the ultimate origins of all reconstructed objects used in physics analyses. They are therefore key to understanding the full kinematics of an interaction. However, increasing luminosity poses a challenge for primary vertex reconstruction in ATLAS. The recently finished Run-2 observed a rate of 60 or more proton-proton collisions per beam crossing, and an even higher vertex density is expected in future. As such, ATLAS has developed new tools: a Gaussian track density seed finder and an adaptive multi-vertex finder. The seed finder locates the position of candidate vertices using an analytic model of the track density along the beam axis, and the vertex finder applies a global approach to vertex finding and fitting that allows charged tracks to be assigned to their optimal vertex candidates. This thesis presents studies of the optimisation and exploitation of these tools for the vertex densities expected in the upcoming Run-3 and beyond.