Search for Supersymmetry in Events with Jets and Missing Transverse Momentum with the CMS Experiment

Abstract

This thesis presents a search for Supersymmetry (SUSY), which is a popular extension of the Standard Model of particle physics (SM). The search is performed in proton-proton collision data in final states with jets and a large transverse momentum imbalance.The analyzed data were collected with the CMS experiment at the CERN LHC in 2016 at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 35.9 fb${}^{-1}$. In the analysis, the number of jets and tagged bottom quark jets, as well as the transverse and missing transverse momentum are used to categorize the events into exclusive search regions. This approach provides a high sensitivity to a variety of supersymmetric final states.The focus of this thesis is on a data-driven estimate of the so-called lost-lepton background. In these events, a neutrino is produced and the associated electron or muon is not observed as an isolated lepton or track, and the event enters the search region. This is one of the dominant background contributions, especially in most sensitive search regions. The background estimation method is based on events in single lepton control regions, which are selected in data and then used to constrain the background prediction in the search regions. This procedure reduces the dependence of the analysis on simulated event samples. Decisive improvements of the well-established lost-lepton background estimation method are described, thus achieving a crucial reduction of the leading systematic uncertainty compared to previous implementations of the method. The new developments are validated by an independent background estimate. This is performed by a second, less complex approach, which is based on the same single lepton control regions but generally relies on the modeling of the simulation to a larger extent.No evidence for SUSY is found in the analyzed data. Therefore, the results are interpreted in the context of simplified models. All considered models assume the neutralino to be the lightest supersymmetric particle. Limits on the cross section for the pair production of gluinos and squarks are derived for various production and decay scenarios, which correspond to lower limits on the gluino mass as large as 1800-1960 GeV and to lower limits on the squark masses as large as 960-1390 GeV at 95% C.L.The statistical interpretation of searches as the one described in this thesis requires large samples of simulated events. To save computational resources, these events are typically generated with a fast and approximate simulation of the CMS detector, referred to as FastSim. In this thesis, a new and efficient framework for the propagation of particles inside the CMS tracking detector is developed and validated. This algorithm permits the modeling of the recently upgraded CMS pixel detector and is expected to remain a core part of FastSim throughout further upgrades of the experiment.