Single top quark production at 13 TeV with the CMS experiment. From rediscovery to search for rare channels and determination of Higgs boson couplings

With the observation of the Higgs boson in 2012, the last missing particle of the standard model of particle physics was discovered. The standard model has successfully described the subatomic world for half a century, but also predicted newparticles, which have been discovered afterwards at particle colliders. Although the standard model is a complete and self-consistent theory, there are unexplained phenomena in nature, whose existence let one come to the conclusion that the standard model is not the final answer to all fundamental questions in particle physics. Many extensions of the standard model have been predicted, but only experimental evidence will finally decide if one of these theories is realized in nature. The purpose of the Large Hadron Collider (LHC), the biggest machine ever built by mankind, is to probe the standard model up to the highest energies ever achieved under laboratory conditions. This is accomplished by accelerating two reverse proton beams up to beam energies of 6.5 TeV and cross their paths at certain interaction points to induce particle collisions. Sophisticated multi-purpose particle detectors, such as the Compact Muon Solenoid (CMS) experiment, are located at crossing points of the beams to record the signatures of each proton-proton collision. With a mass of roughly the same as a gold atom, the top quark is the heaviest fundamental particle in the standard model. Although mainly produced in top quark-antiquark pairs at the LHC through the strong interaction, the production of single top quarks allows to probe the electroweak sector of the standard model. It is not only possible to measure observables of the electroweak theory with single top quark processes at high precision, but also to search for deviations in predictions and data that could be a hint for physics beyond the standard model. Due to its high mass, the top quark is also an excellent candidate to search for associated production with a Higgs boson, in which the coupling of the Higgs boson to the top quark and other fundamental particles can be determined. After the first data-taking period from 2010 to 2012 at center-of-mass energies of 7 and 8 TeV and a two-year shutdown for upgrades of the accelerator, the LHC started operations again in 2015 with Run II at 13 TeV center-of-mass energy. Since then, the LHC has surpassed its design goal in terms of collision intensity and provided more than a quadrillion proton-proton collisions, from which only a tiny fraction is of interest for single top quark research. This thesis follows the journey of the LHC Run II and the data recorded by the CMS detector from the first collisions in 2015 up to the beginning of the precision era at the end of 2017. In the first chapter, the theoretical foundation of the standard model is provided. The chapter starts with a general introduction, with the focus shifted afterwards on the physics of the top quark and the Higgs boson, as well as their interplay. The second chapter introduces the statistical methods employed for the different analyses in this thesis. In the first part, sophisticated multivariate analysis techniques are outlined that help to identify signal candidates out of background-dominated data. The second part explains the underlying statistical reasoning behind the results. The description of the experimental apparatus, namely the particle collider LHC and the CMS detector, are subjects of interest in the third chapter. The journey of the proton is followed from a simple hydrogen bottle to the collision point and from the decay products of the collisions to the signals in the readout electronics. The fourth chapter first reveals how simulated proton-proton collisions are generated to describe the measured data. In the second part of the chapter, the reconstruction procedure is introduced, which is used to reassemble physics objects from detector signals for simulation and data alike. The purpose of the fifth chapter is to describe a common physics object definition, shared between the different analyses presented in this thesis. In addition, certain quality criteria are defined that ensure proper modeling of the measured data by the predictions. In the sixth chapter, a measurement of the cross section of single top quark production in the t channel is presented. The measurement is based on the first year of proton-proton collision data at a center-of-mass energy of 13 TeV in 2015. The t channel is the most dominant production mode of single top quarks at the LHC and is therefore well suited for a first single top quark measurement at Run II of the LHC. With the increased amount of data recorded during Run II, the focus shifts to more rare single top quark production channels. One of these channels is the s-channel single top quark production, the only major single top quark production mode yet unobserved at the LHC. The seventh chapter is dedicated to the search for this channel. To increase the sensitivity of this search, the combined data sets of 2016 and 2017 are used. In the eighth chapter of this thesis, a search for the associated production of a Higgs boson with a single top quark is presented. This associated production mode is highly sensitive to the coupling of the Higgs boson to the top quark and to vector bosons. The final chapter summarizes the three different single top quark analyses, presents a conclusion and provides an outlook for the promising field of single top quark research.