Performance of detector modules for the CMS Pixel Phase 1 upgrade and search for $t\overline{t}H, H \rightarrow b\overline{b}$ events in the boosted regime

Abstract

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.