Many theories beyond the Standard Model predict new massive particles that decay pref- erentially to top-antitop quark pairs. This thesis investigates the dilepton final state where both W bosons from the top quarks decay into leptons. This final state features the highest purity for top quarks but is kinematically under-constrained due to the presence of the two neutrinos that leave the ATLAS detector undetected. Using kinematic constraints from the top-antitop quark decay chain, along with the missing energy, leptons, and jets, that are measured with the ATLAS detector, a set of analytic solutions for the four-vectors of the neutrinos can be obtained. With this information, the invariant mass can be reconstructed up to a four-fold ambiguity. Methods used to eliminate incorrect neutrino solutions are in- vestigated and characterized. For a resonance particle with a mass much larger than the top quark mass, the top quarks will be highly boosted, resulting in collimated decay products of the top quark. Optimizing the lepton selection in such an environment is very important and is also discussed.