Multijet Background Estimation For SUSY Searches And Particle Flow Offline Reconstruction Using The ATLAS Detector At The LHC

This thesis describes the jet smearing method, a data-driven technique for estimating the multijet background to Supersymmetry (SUSY) searches using the ATLAS detector at the Large Hadron Collider (LHC). The final 2011 and 2012 “ATLAS jets, missing transverse energy and zero leptons analysis” searches for SUSY are also documented. These analyses used the full ATLAS 2011 4.7 fb^{-1} $\sqrt{s}$ = 7 TeV and 2012 20.3 fb$^{-1}$ $\sqrt{s}$ = 8 TeV data sets. No statistically significant excess was found in either of these analyses; therefore, 95% C.L. mass exclusion limits were set on the mSUGRA/CMSSM m$_{0}$-m$_{1/2}$ and $m_{\tilde{q}}$-$m_{\tilde{g}}$ mass planes, and the simplified squark-gluino-neutralino pMSSM model. The jet smearing method was used in these analyses to estimate the multijet distributions of the Signal, Validation and Control Regions and also to calculate the multijet background Transfer Factors. This thesis also describes the missing transverse energy (E$_{miss}^{T}$ ) performance studies of the ATLAS particle flow (PFlow) offline reconstruction algorithm in Z->$\mu\mu$, $t\bar{t}$ and Wt events using Monte Carlo simulated data and the ATLAS 2012 $\sqrt{s}$ = 8 TeV data set. The particle flow E$^{T}_{miss}$ algorithms out-performed MET RefFinal in most aspects, including resolution, scale and pile-up independence. The particle flow algorithm also described the expected $M_{T}$ distribution in $t\bar{t}$ and Wt events better than MET RefFinal.