Search for Supersymmetry with Multiple Charged Leptons at sqrt{s} = 13 TeV with CMS and Radiation Tolerance of the Readout Chip for the Phase I Upgrade of the Pixel Detector

Abstract

The present dissertation documents contributions to two complementary aspects of high- energy particle physics research with the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). The work includes an analysis of proton-proton collisions to search for physics beyond the standard model (SM) and radiation tolerance assurance studies conducted in the context of the development of a new pixel detector. The first part of the thesis describes a search for so-far undiscovered physics processes, which are predicted for example by supersymmetric extensions of the SM. To this end, events with at least three charged leptons as well as hadronic jets and missing transverse momentum are analyzed at the unprecedented center-of-mass energy of 13 TeV. The study is based on a data set corresponding to an integrated luminosity of 12.9 fb^-1 of proton-proton collisions that has been recorded with the CMS detector in 2016. The examined final state features very low SM background contributions. The resulting high sensitivity for different beyond the SM processes can be further enhanced by categorizing events of interest into 32 exclusive signal regions. Reducible and irreducible SM background contributions to these signal regions are estimated with data and Monte Carlo simulations, respectively. The observed data are found to agree with the SM predictions within the assigned uncertainties. The results are therefore interpreted in terms of exclusion limits on masses of supersymmetric particles in the context of simplified model topologies that feature gluino pair production and a neutralino as the lightest supersymmetric particle (LSP). In a model producing four top quarks and two LSPs, gluino masses up to 1 200 GeV could be excluded, extending the exclusion limit set by a similar search at sqrt{s} = 8 TeV by about 200 GeV. Gluino masses up to 1 000 GeV could be excluded in a model with light-flavor jets, vector bosons, and two LSPs in the final state. The second part of the thesis is dedicated to irradiation studies with the new readout chip (ROC) for the Phase I Upgrade of the CMS pixel detector. The replacement of the pixel detector with an improved version has been completed in February 2017 and aims at main- taining and improving the performance of the detector at increased instantaneous luminosities of up to 2 × 10^34 cm^-2 s^-1 . One of the key components of the new detector is a revised ROC, which is used in the forward part and in layers 2-4 of the central detector, designed for hit detection efficiencies above 99% at pixel hit rates up to 120 MHz/cm^2 . Because of the close proximity to the collision point, the new ROC is faced with stringent demands in terms of tolerance against ionizing radiation damage. Studies conducted in the context of this disser- tation significantly contributed to improving the radiation tolerance of the chip during its development phase. Moreover, it could be confirmed that the performance of the final ver- sion of the chip meets the design specifications even after accumulating the expected lifetime doses for layers 2 and 1 of 0.5 MGy and 1 MGy, respectively. Several important performance figures of the ROC have been shown to comply with the specifications even after heavy irradi- ation of up to 4.2 MGy. The thesis not only describes comprehensive functionality tests after irradiation, but also provides recommendations for dose dependent adjustments of operation parameters that will be necessary to optimize the performance of the chip in the experiment.