Measurement of the top quark mass with the matrix element method in the semileptonic decay channel at D0

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The top quark plays a special role in the Standard Model of Particle Physics. With its enormous mass of about 170 GeV it is as heavy as a gold atom and is the only quark with a mass near the electroweak scale. Together with theW boson mass, the top quark mass allows indirect constraints on the mass of the hypothetical Higgs boson, which might hold the clue to the origin of mass. Top pair production with a semileptonic decay t $\bar{t}$ →W±W∓ b$\bar{b}$ →q $\bar{t}$lnb$\bar{b}$ is the ”golden channel” for mass measurements, due to a large branching fraction and a relatively low background contamination compared to other decay channels. Top mass measurements based on this decay, performed with the matrix element method, have always been among the single best measurements in the world. In 2007, the top mass world average broke the 1% level of precision. Its measurement is no longer dominated by statistical but instead by systematic uncertainties. The reduction of systematic uncertainties has therefore become a key issue for further progress. This thesis introduces two new developments in the treatment of b jets. The first improvement is an optimization in the way b identification information is used. It leads to an enhanced separation between signal and background processes and reduces the statistical uncertainty by about 16%. The second improvement determines differences in the detector response and thus the energy scales of light jets and b jets. Thereby, it addresses the major source of systematic uncertainty in the latest top mass measurements. The method was validated on Monte Carlo events at the generator level, calibrated with fully simulated events, including detector simulation, and applied to D0 Run II data corresponding to 1 fb-1 of integrated luminosity. Possible sources of systematic uncertainties were studied. The top mass is measured to be: mt = (169.2±3.5(stat.)±1.0(syst.)) GeV . The simultaneous measurement of a scaling factor for the jet energy scale of light jets and a separate scaling factor for b jets yields 1.038±0.023 and 1.056±0.045, respectively. This result indicates that the nominal D0 jet energy scale derived from γ+jets events underestimates the energy of light jets and b jets in t $\bar{t}$ decays. The improved analysis was successful in reducing the major systematic uncertainty caused by the b jet energy scale from about 800 MeV to approximately 150 MeV.