Measurements of rare processes with top quarks and electroweak bosons in proton-proton collisions at the CMS detector

Introduction This dissertation studies several rare elementary particle processes involving top quarks, in an attempt to test and challenge our current knowledge of the fundamental workings of the Universe. The standard model of particle physics is a comprehensive and quantitative description of all known elementary particles and their interactions. Despite its tremendous success in accurately predicting probabilities and characteristics of elementary particle processes, tested and verified in many experiments, it is known to be incomplete. This dissertation fits in the context of pushing the standard model to its limits, by studying rare processes involving the heaviest known elementary particle, the top quark, with the aim of finding clear deviations from the prediction, to be interpreted either as signs of new physics effects or of specific modeling issues. Data The analyses in this dissertation were performed on data recorded with the CMS detector at the CERN LHC. The Large Hadron Collider (LHC) is currently the world's largest and most powerful particle accelerator and collider. The Compact Muon Solenoid (CMS) is a composite detector that aims to detect the outgoing particles and reconstruct the underlying fundamental interaction of the proton-proton collisions in its center. The data sample used in this work was collected by CMS in the years 2016–2018 and consists of 138 fb−1 of proton-proton collisions at a center-of-mass energy of 13 TeV. Processes In particular, this dissertation focuses on tZq (simultaneous production of a single top quark and a Z boson) and ttW (production of a top quark pair together with a W boson). Both processes are rare because of the heavy final state and the electroweak interaction(s), and are expected to be sensitive to several scenarios for extensions of the standard model. The tZq process is particularly interesting because of its purely electroweak production mode and direct coupling between the top quark and Z boson. The ttW process is notoriously difficult to model because of the large electroweak and higher order QCD corrections. For tZq, we obtain a total uncertainty in the cross section of about 11%, which makes this the most precise measurement of the tZq cross section so far. The measurement is in agreement with the standard model prediction within experimental and theoretical uncertainties. In the case of ttW, we perform differential cross-section measurements as a function of several variables such as the kinematic properties of the leptons and jets in the events. The results of these measurements can be used as a reference to test modeling improvements against. This analysis is currently in CMS internal review, and therefore the results shown in this work are expected outcomes based on simulation. Lepton identification In both the tZq and ttW analyses, a vital ingredient is the accurate identification of leptons (electrons and muons), and the estimation of background processes with fake or secondary leptons. The extensive research reported in this dissertation regarding the estimation of this background constituted an important contribution to several other related measurements, such as the simultaneous production of four top quarks and a search for heavy neutral leptons. Displaced vertexing calibration In addition to the above, this dissertation contains a study of displaced tracking and vertexing. This study is carried out in the context of searches for long-lived heavy neutral leptons or other exotic experimental signatures involving hypothesized particles with relatively long lifetimes, decaying at macroscopic distances from the proton-proton interaction point. The reconstruction efficiency of these displaced particles is typically estimated from simulation, but is difficult to calibrate. In this dissertation, we use the decay of the neutral K-meson to two pions to assess the accuracy of the simulation of displaced tracking and vertexing and to derive calibration factors. The reconstruction of displaced K-meson vertices is compared between data and simulation, as a function of the radial displacement distance. We obtain calibration factors generally in the order of 10–15% for large displacements, with a strong dependency on dynamic detector inefficiencies. The study is performed for several reprocessing versions of the CMS data and simulation. In particular, we observe the strong impact of an issue with the tracker readout electronics in the early half of 2016 data taking, which is reduced by a reprocessing of the simulation. Automatic data quality monitoring Finally, this dissertation shows a few case studies of applying machine learning to DQM and DC. Data quality monitoring (DQM) and data certification (DC) consist of ensuring the good quality of data recorded and reconstructed by CMS, and are vital for its correct interpretation and for the reliability of physics analyses using it. We apply machine learning tools, in particular (convolutional, residual) autoencoders to automatically detect potential detector issues in the CMS pixel tracker, with a finer time granularity than would be feasible with the conventional techniques. We show that an anomaly detection efficiency of essentially 100% can be achieved, with a sufficiently low false alarm rate, though the test set to obtain these numbers cannot be unambiguously defined. Furthermore, important technical advances were made to provide the quality monitoring data with this finer time granularity as a standard workflow in CMS.