Direct measurement of the top quark decay width in the muon + jets channel using the CMS experiment at the LHC

This thesis investigates the decay properties of the top quark. In the SM the top quark decays almost exclusively into a W boson and a b quark. The probability for this process to happen is reflected in the top quark decay width, which is predicted to have a value around 1.33 GeV. If the top quark is able to decay into other particles as well, as is possible in several extensions of the SM, the top quark decay width will be larger than the SM prediction. This is investigated by performing a direct measurement of the top quark decay width using proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of 13 TeV. The data were recorded by the CMS experiment in 2016 and correspond to an integrated luminosity of 35.9 fb−1 . At first, a concise overview of the standard model is presented in Chapter 1. The production and decay of the top quark is described in more detail and the current status of the top quark mass and decay width measurements is given. Top quarks are typically studied by producing them in particle colliders. Chapter 2 describes the Large Hadron Collider and the Compact Muon Solenoid experiment, which were used to produce and record the top quark events analysed in this thesis. In order to understand how the observations in a particle detector correspond to the theoretical interactions between colliding particles, collision events are simulated. A step-by-step overview is given in Chapter 3. In addition, the simulated samples used in this thesis are summarised and a reweighting technique is presented to acquire simulated top quark pair events with a distribution characterised by an arbitrary top quark mass and decay width. Chapter 4 expands on how electronic detector signals are processed and combined to reconstruct particles. These are used in Chapter 5 to set up a basic event selection and reconstruct higher-order objects, such as the top quark. Based on the reconstructed event topology, additional requirements are posed on the selected events. The measurement procedure itself is based on a maximum likelihood technique and is described in Chapter 6. It is set up using simulated events only, so as not to let the result of the measurement influence the procedure. Probability density functions are constructed using the distributions of variables that are sensitive to changes in the top quark decay width. These changes are simulated using the reweighting technique introduced in Chapter 3. Further, the measurement procedure is calibrated and the systematic uncertainties affecting the measurement are discussed. In addition, the likelihood functions of several sensitive variables are combined to get a more precise measurement. The results of the two top quark decay width measurements presented in this thesis are summarised in Chapter 7 and possible improvements to obtain an even more precise measurement are discussed.