Constrained Kinematic Fitting for a Top Quark Mass Determination in the Electron + Jets Channel at ATLAS

For the measurement of top-quark properties a high purity of the selected events is indispensable. From this perspective the most promising $toverline{t}$-decay channel is the electron + jets channel, e. g. $toverline{t} ightarrow W^{+}bW^{-}overline{b} ightarrow q overline{q}'b ell overline{ u} overline{b}$. In the current standard reconstruction method, $p_{mathrm{t,max}}$, the jet triplet with the highest transverse momentum is used to reconstruct the properties of the hadronically decaying top-quark. The prospects for a kinematic fitting approach with constraints on the W mass and on the difference of the masses of the two top-quarks are presented. The method is tested with Monte Carlo events simulated at next-to-leading order with MC@NLO. Results are obtained, both at the level of partons (parton level) as well as for objects obtained after detector simulation and reconstruction (detector level). At detector level additional contributions from W + jet and multijet background events are investigated. The selection efficiency and the improvement on top-quark mass measurements are reviewed. Also the prospects for a b-jet identification based on the selection presented in this thesis is laid out.