Jet response at ATLAS using the Missing Projection Fraction Technique: W+jets background and validation at high energy using multijets.

Experiments at the LHC put much effort into knowing the uncertainty on the jet energy scale (JES) at high pT because it is the largest source of systematic uncertainty in many important analyses. One method for deriving the energy scale correction due to calorimeter response is the Missing ET Projection Fraction (MPF) technique, which utilizes balance in transverse momentum between photon and jet in direct photon events. The momentum range of this method increases with the number of gamma+jet events recorded. Multijet events can be used to check the consistency of the MPF response at high momentum scales, taking advantage of the large QCD cross-section. This thesis explores pT balance with multijets using a Monte-Carlo based, flavor-corrected MPF response at ATLAS. In addition, the very small W+jets background to gamma+jet selection is considered.