1. Introduction -- 2. Experimental Environment -- 3. Event Reconstruction and Simulation -- 4. The Top Quark and the Concept of Mass -- 5. The Matrix Element Method -- 6. Measurement of the Top Quark Mass -- 7. Improved Mass Measurement -- 8. Conclusion -- A Solving for the Event Kinematics -- B The Jacobian Determinant for the Signal Integration.

The top quark, discovered in 1995 at the Fermilab Tevatron Collider, is the heaviest known elementary particle. The precise knowledge of its mass yields important constraints on the mass of the as-yet-undiscovered Higgs boson and allows one to probe for physics beyond the Standard Model. With an excellent adaptation of a novel measurement technique, described and applied here for the first time, the sensitivity to the top quark mass in the dilepton final state at the D0 experiment could be improved by more than 30%. Moreover, an extension to the method is presented which allows future measurements to significantly reduce the main limiting systematic uncertainty.