ii ABSTRACT A measurement of the top quark mass with the matrix element method in the lepton + jets final state in DØ Run II is presented. Events with single isolated energetic charged lepton (electron or muon), exactly four calorimeter jets, and significant missing transverse energy are selected. Probabilities used to discriminate between signal and background are assumed to be proportional to differential cross-sections, calculated using event kinematics and folding in object resolutions and parton distribution functions. The event likelihoods constructed using these probabilities are varied with the top quark mass, m t , and the jet energy scale, JES, to give the smallest possible combined statistical + JES uncertainty. The results using 425 pb Also presented is an overview of the installation of several scintillation detectors in the DØ muon system, and a comprehensive calibration of the muon scintillation system. iii ACKNOWLEDGMENTS