Search for Supersymmetry in Opposite-sign Dilepton Final States with the CMS Experiment

For centuries, men’s curiosity drove him to explore the secrets of nature and the essentialprinciples and constituents of matter. Today, the Standard Model of particle physicssuccessfully describes a significant part of the known universe and most experimentalobservations. Still several experimental and theoretical findings indicate that the StandardModel is only valid up to an energy scale of a few TeV.The Large Hadron Collider is, together with its detector experiments, the largest and mostsophisticated experiment ever constructed and conducted in the history of mankind. It isdesigned to collide protons at a centre-of-mass energy of up to 14 TeV, and its purpose isto test the current Standard Model of particle physics and search for physics beyond thismodel. One promising new-physics theory is Supersymmetry, which extends the StandardModel by supersymmetric partner particles for the particles in the Standard Model.This work uses proton-proton collision data at a centre-of-mass energy of 7 TeV taken bythe Compact Muon Solenoid experiment at the Large Hadron Collider to search for signsof Supersymmetry. A special decay mode of a neutralino particle is targeted, which produces two flavour-correlated leptons and a stable supersymmetric particle that escapes theexperiment undetected. The decay mode results in a characteristic edge in the invariantmass distribution of the two produced leptons. This characteristic mass edge is used asa search criterion for physics beyond the Standard Model, and a way to separate thisedge from background produced by Standard Model processes is presented. Separation isachieved by performing a fit to the invariant-mass distribution of events with two leptons.The fit shape is constructed using a background model, which is determined from data,and a signal hypothesis based on the mass edge that is produced by the target decay. Thisapproach does not rely on simulated data, all backgrounds are estimated directly fromdata.The search method is tested on simulated data with and without a hypothetical signal,resulting in a good performance. Afterwards, the method is performed on data. Thedataset, recorded in the year 2011, corresponds to an integrated luminosity of 4.98 fb−1 .No signs of physics beyond the Standard Model are found. Therefore, exclusion limits onnon-Standard-Model physics are calculated. These limits are interpreted in the contextof a simplified model that incorporates the targeted decay and the constrained minimalsupersymmetric extension of the Standard Model.This work is structured in the following way: an introduction into the theoretical foundations on which this work is based is given in Chapter 1. Chapter 2 describes the CompactMuon Solenoid experiment, whose data serves as input to this analysis. In the followingchapter, Chapter 3, the data taking and production of simulated data is described as wellas the procedure for reconstruction of usable physics objects from data and simulateddata. In Chapter 4, the search method is detailed. This includes the chosen event selection, a description of the expected Standard Model backgrounds, methods to estimate thevarious background contributions, the performance of the method on simulated data andfinally the search results on actual data. These results are interpreted in Chapter 5. Since