Measurement of the Z boson cross-section in - Harvard University ...

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Chapter 1: Introduction and Theoretical Overview 4 theoretical calculation of W/Z cross-sections. In Chapter 2, we briefly describe the LHC and the various subsystems of the ATLAS detector. Chapter 3 contains a detailed discussion of the measurement of luminosity by the LHC as well as by the ATLAS detector. Details of data collection and event reconstruction can be found in Chapter 4, together with the measurement of muon reconstruction efficiency. Chapter 5 summarizes the Monte Carlo generators and samples we use in the analysis. We discuss Z event selection criteria and related acceptance and efficiencies in Chapter 6. In Chapter 7, we study the principal backgrounds to our signal channel and the procedures adopted to minimize them. In Chapter 8, we show the results of the Z boson selection and the measurement of the Z cross-section. Finally, in Chapter 9, we compare our results with theoretical predictions. In this chapter, we also show properties of Z bosons obtained with a data sample larger than that used for the analysis. Note that all data distributions shown in this thesis have statistical error bars only unless otherwise specified. The size of the error bars corresponds to a 68.3% confidence interval. Also, the term ‘Z → µµ cross-section’ will refer to the cross- section of Z/γ ∗ production multiplied by the branching fraction (Z/γ ∗ → µ + µ − ), unless otherwise stated. 1.2 The Standard Model The Standard Model of particle physics describes particles and their interactions in terms of at least 19 free parameters [45]. The Model is invariant under local transformations of the gauge group SU(3)C × SU(2)L × U(1)Y . It contains three
Chapter 1: Introduction and Theoretical Overview 5 families of quarks and leptons, which are massive spin- 1 2 fermions, as well as spin-1 (vector) bosons which mediate the electroweak and strong interactions. The Standard Model does not incorporate gravity. Quantum Electrodynamics (QED), which was developed in the 1940s and 50s, describes electromagnetic interactions in terms of an exchange mechanism [42]. It requires the existence of a massless gauge boson, i.e. , the photon. In the 1970s, the electromagnetic interaction was successfully unified with the weak interaction [108], which is mediated by the massive vector bosons W ± and Z 0 . The electroweak theory is invariant under SU(2) L ×U(1) Y transformations, its couplings being parametrized by two dimensionless constants. Quantum Chromodynamics, described by the group SU(3)C, contains an octet of massless gluons as force mediators. Unlike the mediators of the electroweak force, gluons can interact among themselves. A consequence of this self-interaction is that the QCD coupling αs decreases as the energy scale of the interaction increases, leading to the so-called asymptotic freedom of the theory [109]. Gluon self-interaction also means that colored objects such as free quarks and gluons cannot be observed; only colorless bound states are experimentally observable. In the electroweak sector, the quarks are able to mix via the Kabibbo-Kobayashi- Maskawa (CKM) mixing matrix, which contains three mixing angles and a phase [22]. A similar mixing matrix exists in the neutrino sector, containing three angles and three phases (e.g., see [9] and references therein). Mixing is also possible in the QCD sector, but the mixing parameter in this case very close, if not identical, to zero (e.g., [37]).
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