Maren Tabea Meinhard

The observation of Higgs boson production in association with a top quark-antiquark pair is reported, based on a combined analysis of proton-proton collision data at center-of-mass energies of √s = 7, 8, and 13 TeV, corresponding to integrated luminosities of up to 5.1, 19.7, and 35.9  fb^(-1), respectively. The data were collected with the CMS detector at the CERN LHC. The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance from the standard model for a Higgs boson mass of 125.09 GeV is 4.2 standard deviations. The combined best fit signal strength normalized to the standard model prediction is 1.26^(+0.31)_(−0.26).

The first layer of the CMS Phase 1 pixel detector will be located at a distance of 3 cm from the interaction point. Pixel hit rates up to 600 MHz/cm2 are expected at the instantaneous luminosity of 2×1034 cm−2 s−1 foreseen by LHC in the coming years. The CMS Phase 1 pixel detector will be in operation until 2024/25 and the total fluence received by the first layer in its lifetime will reach 2–3×1015 neq/cm2 that corresponds to 0.8–1.2 MGy. A new readout chip, called PROC600, to be used for layer 1 modules has been designed at PSI. To validate robust and efficient operation of PROC600, it has been irradiated to doses ranging from 0.6 MGy up to 4.8 MGy. The chip performance before and after irradiation including the pixel hit efficiency will be presented.

The instantaneous luminosity of the Large Hadron Collider will increase to up to 2x1034\;cm−2s−1 by 2023. In order to cope with such luminosities, the pixel detector of the CMS experiment has been replaced in January 2017. The upgraded detector features four sensitive layers in the barrel part. A designated readout chip (PROC600V2) is used for layer 1, which is located only 3 cm from the interaction point and therefore has to handle large particle fluxes. An irradiation campaign has been performed with PROC600V2 to verify its radiation tolerance up to the maximum expected dose for 2017 of 0.2 MGy which proved that no performance loss is expected at the tested doses. Modules for layer 1 have been built with PROC600V2 for the detector production. The quality of every inserted module was assessed in a number of tests, some of which were performed using X-radiation. The characteristics of the modules used in the detector as well as the main failure modes will be presented. It will be shown that the installed modules have an efficiency of over 98% at the maximum expected particle hit rate in CMS.

In high energy particle physics, accelerator- and detector-upgrades always go hand in hand. The instantaneous luminosity of the Large Hadron Collider will increase to up to L = 2×1034cm−2s−1 during Run 2 until 2023. In order to cope with such luminosities, the pixel detector of the CMS experiment has been replaced early 2017. The so-called CMS Pixel phase 1 upgrade detector consists of 1184 modules with new design. An important production step is the module qualification and calibration, ensuring their proper functionality within the detector. This paper summarizes the qualification and calibration tests and results of modules used in the innermost two detector layers with focus on methods using module-internal calibration signals. Extended characterizations on pixel level such as electronic noise and bump bond connectivity, optimization of operational parameters, sensor quality and thermal stress resistance were performed using a customized setup with controlled environment. It could be shown that the selected modules have on average 0.55‰ ± 0.01‰ defective pixels and that all performance parameters stay within their specifications.

This contribution outlines the implementation of the matrix element method (MEM) in the search for $\text{t}\bar{\text{t}}$H, H $\rightarrow \text{b}\bar{\text{b}}$ events. In particular, the evaluation of the transfer functions, which relate detector level to parton level quantities in the computation of the MEM, is described. In addition, it is presented how jet substructure reconstruction can be combined with the MEM. The combination of these techniques leads to a decrease in computation time by up to 90% and an increase in event selection efficiency of 30% in the high Higgs boson and top quark p$_{T}$ phase space.

This thesis presents two contributions to the research programme in particle physics of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). The increasingly stringent operating conditions of the LHC accelerator led to the replacement of the innermost component of the CMS experiment, its pixel detector, with an upgraded version in early 2017. This improved its performance and allowed operation up to instantaneous luminosities of 2 ×1034 cm−2 s−1. The functionality of the readout chip used for the innermost layer of the detector has been investigated in this thesis, in particular its tolerance towards radiation damage. For this purpose, the characteristics of the readout chip have been studied at increasing irradiation doses. This proved that no property of the chips is problematically altered by irradiation, and provides essential calibration parameters to successfully operate the chips in the experiment throughout their expected lifetime. Moreover, the quality of the modules built from these readout chips for the final detector has been monitored during the course of this work. This allowed to identify and reject modules with defects such as clusters of irresponsive pixels or faulty connections between the sensor and the readout electronics. When measuring the properties of the Higgs boson with the highest possible accuracy, the t tH, H→b b process is of particular interest, as it allows a direct mea- surement of the Yukawa coupling between the top quark and the Higgs boson. This work focuses on a search for such events where either of these particles are produced at large transverse momentum (pT ), which is a region of phase space for which many theories predict deviations from Standard Model expectations. Two different dis- criminators are studied in this context. The matrix element probability is a powerful variable to differentiate between the signal and the main background, t t+b b. During this thesis a numerical instability during its computation was discovered, however a promising method to use this discriminant in a search for events with objects pro- duced at large pT is presented nonetheless. Another approach has been developed, that uses a set of kinematics and b tagging related variables, to measure a best-fit signal strength for the t tH, H→b b process of 0.47+3.34−3.41. This result is largely driven by events with objects produced at lower pT . Events with a Higgs bosons or a top quark with a larger pT are expected to improve the analysis sensitivity when using state of the art identification methods for these objects.