Estimation of QCD Multijet Background for Top Antitop Events from Data

Precision measurements of the cross section for top-quark pair production are an important test of the Standard Model. The Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) will collect t ̄ events with high efficiency. The purity of such data is then limited by the understanding of the shape and ratio of background processes. Besides W+jets events QCD multijet events are an important background for the process pp → tt + X → μνbbqq + X. The Matrix Method, originally developed at the D∅ experiment at the Fermilab Tevatron, provides a possibility to estimate the QCD multijet background from data. In the original version of the method a high-purity QCD sample is extracted from a low missing transverse energy region in order to determine the isolation efficiency of the lepton. In the next step the isolation efficiency is used to estimate the background in a signal region. Within this thesis the Matrix Method has been adapted to the CMS experiment. Instead of missing transverse energy, a multivariate control variable is used to define a control region with minimal signal contamination. The following uncertainties of the method on the estimated number of QCD events after muon isolation have been studied: the relative normalization of signal and QCD events, the shapes of the input variables and the choice of the control region.