Search for Pair Production of Supersymmetric Top-Quark Partners in Events with a Single Lepton at CMS

The analysis presented in this thesis is a search for direct pair production of supersymmetric top-quark partners at CMS.Supersymmetry is a compelling theory providing possible solutions to several of the Standard Models limitations. However, previous searches for supersymmetric particles came backwith empty hands. These results and the discovery of a Higgs boson with a mass of about125 GeV by the ATLAS and CMS Collaborations strongly constrain the simplest supersymmetric models. Nevertheless, more sophisticated models with light third-generation squarksdid not lose their theoretical appeal and are within the reach of the 8 TeV run of the LargeHadron Collider.In this analysis, a search for direct top-squark (˜) pair production is performed in a finaltstate consisting of a single isolated lepton, jets, among which at least one is a b-tagged jet,and large missing transverse energy. Six search regions are defined with a semi-automaticprocedure to maximize the sensitivity of the analysis. The background estimation is performed using simulated samples validated in control regions with small or no signal contamination. Scale factors are measured in the control regions and used to correct the backgroundin the search regions if needed. The observed event yields in the search regions agree withthe predicted backgrounds within the uncertainties, hence no evidence for pair-producedtop-squarks can be inferred. The results are used to constrain top-squark pair production inthe framework of simplified models. Two possible top-squark decay modes are considered:the decay to top quark and a neutralino ( chiz), ˜ → t χ0 , and the decay to a bottom quarktand a chargino (χ+ ), ˜ → b χ+ , with the subsequent χ+ → W + χ0 decay. Exclusion limitstare set for branching ratios B(˜ → t χ0 ) = 100% and B(˜ → t χ0 ) = 50%. In the formerttcase, for small mass values of the lightest neutralino, the analysis probes top-squark massesup to 600 GeV and up to 500 GeV in the mixed decay model. The analysis presented inthis thesis is the first to be able to exclude several model points with m˜ − mχ0 ≤ mt fortB(˜ → t χ0 ) = 50%, restricting the allowed parameter space of natural SUSY scenarios.t