The field of particle physics involves not only searches for new particles and measurements of their interactions, but also the design and construction of advanced particle detectors.This thesis presents the measurement of the production cross section of four top quarks in proton-proton collisions at a center-of-mass energy of 13 TeV using 137 fb−1 of integrated luminosity recorded by the CMS experiment at the LHC. This analysis considers events in the final state of a same-sign pair of leptons, notable for being a final state with relatively few Standard Model background events. A boosted decision tree is utilized to discriminate four top quark events from background events. The four top quark production cross section is measured to be 12.6+5.8−5.2 fb, consistent with the Standard Model prediction. This measurement is used to constrain the top quark's Yukawa coupling as well as various theories beyond the Standard Model.This thesis also describes the construction and testing of silicon pixel detector modules used in the Phase I upgrade of the CMS pixel detector, and optimization of electron reconstruction methods using the new detector. The role of automated module assembly and quality assurance will be discussed, as well as work towards the construction of a high precision silicon strip detector based telescope.

The top quark pair production cross section is measured for the first time in proton-proton collisions at √s= 13 TeV by theCMS experiment at the CERN LHC, using data corresponding to an integrated luminosity of 42 pb-1. The measurement is performed by analyzing events with at least one electron and one muon of opposite charge, and at least two jets. We then measured the cross section and found that was 769 ± 60 (stat) ± 55 (syst) ± 92 (lumi) pb, in agreement with the expectation from the standard model.

This thesis presents the first experimental calibration of the top-quark Monte-Carlo mass. It also provides the top-quark mass-independent and most precise top-quark pair production cross-section measurement to date. The most precise measurements of the top-quark mass obtain the top-quark mass parameter (Monte-Carlo mass) used in simulations, which are partially based on heuristic models. Its interpretation in terms of mass parameters used in theoretical calculations, e.g. a running or a pole mass, has been a long-standing open problem with far-reaching implications beyond particle physics, even affecting conclusions on the stability of the vacuum state of our universe. In this thesis, this problem is solved experimentally in three steps using data obtained with the compact muon solenoid (CMS) detector. The most precise top-quark pair production cross-section measurements to date are performed. The Monte-Carlo mass is determined and a new method for extracting the top-quark mass from theoretical calculations is presented. Lastly, the top-quark production cross-sections are obtained – for the first time – without residual dependence on the top-quark mass, are interpreted using theoretical calculations to determine the top-quark running- and pole mass with unprecedented precision, and are fully consistently compared with the simultaneously obtained top-quark Monte-Carlo mass.

The cross section for the production of single top quarks in the $t$ channel is measured in proton-proton collisions at 13 TeV with the CMS detector at the LHC. The analyzed data correspond to an integrated luminosity of 2.3 fb$^{-1}$. The event selection requires one muon and two jets where one of the jets is identified as originating from a bottom quark. Several kinematic variables are then combined into a multivariate discriminator to distinguish signal from background events. A fit to the distribution of the discriminating variable yields a total cross section of 232 $\pm$ 13 (stat) $\pm$ 28 (syst) pb and a ratio of top quark and top antiquark production of $R_{t\textrm{-ch.}}= $ 1.81 $\pm$ 0.18 (stat) $\pm$ 0.15 (syst). From the total cross section the absolute value of the CKM matrix element $V_{\mathrm{tb}}$ is calculated to be 1.03 $\pm$ 0.07 (exp) $\pm$ 0.02 (theo). All results are in agreement with the standard model predictions.

Before any kind of new physics discovery could be made at the LHC, a precise understanding and measurement of the Standard Model of particle physics' processes was necessary. The book provides an introduction to top quark production in the context of the Standard Model and presents two such precise measurements of the production of top quark pairs in proton-proton collisions at a center-of-mass energy of 7 TeV that were observed with the ATLAS Experiment at the LHC. The presented measurements focus on events with one charged lepton, missing transverse energy and jets. Using novel and advanced analysis techniques as well as a good understanding of the detector, they constitute the most precise measurements of the quantity at that time.

The search for the production of $\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}$ in $pp$ collisions at $\sqrt{s}\,=\,13\,$TeV is presented in this thesis for the full \lhc \runii dataset of 139$\,$fb$^{-1}\,$collected by the ATLAS experiment at CERN. With an expected cross-section of $\sigma_{\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}}\,=\,11.97^{+18\%}_{-21\%}\,\text{fb}$, the process is one of the rarest being studied at the \lhc. This document focuses on high jet and b-jet multiplicities with one or two leptons of opposite charge. A Boosted Decision Tree and the sum of the transverse mo...

The associated production of a W boson and a single charm quark (W+c) is the only process in proton-proton collisions that directly probes the strange quark content of the proton. In this thesis, W+charm production is measured in proton-proton collisions at the LHC at 13 TeV, as recorded by the Compact Muon Solenoid (CMS) experiment. The analysis focuses on the identification of W bosons in their leptonic decay to a muon and a neutrino and charm quarks are tagged via the full reconstruction of D*-Mesons. The measured cross sections of W+c production are used, in combination with other relevant CMS results and the most precise HERA DIS data, in a QCD analysis to determine the strange quark content of the proton. The resulting strange quark distribution and suppression, with respect to the other light sea quarks, are in good agreement with those obtained in neutrino scattering experiments and extend their kinematic reach.