Top Mass Determination in the ttbar Semileptonic Decay Channel

The proton proton collider LHC will operate at sqrt( s)=14TeV center of mass energy. The NLO cross section for the production of ttbar events reachs at this energy about 800pb. This leads, already for a start-up luminosity of L=10^33 cm^-2 s^-1, corresponding to an integrated luminosity of 10fb^-1, to the huge production rate of 8*10^6 ttbar events per year. This will result in 3.5*10^6 ttbar events in the semileptonic decay channel ttbar -> b W^+ bbar W^- -> b bbar q qbar l neutrino. This signature is used for simulations, including the response of the CMS detector, to determine the top mass in the hadronic decay branch via the invariant three jet mass m( jjj). The invariant mass m( jj) of the light quark jets is scaled to the known W boson mass. The top quark mass is then reconstructed by the invariant mass of the three jet system consisting of a b jet and the scaled light quarkjets. After the event selection criteria are applied a background free sample of about 5000 events remains. This leads to a statistical error of about 250MeV in the top mass. The dominating systematic errors depend on the uncertainties of the top pt spectrum due to higher order calculations, on the achievable accuracy of the detector, e.g. the uncertainty on the calibration of the absolute calorimeter energy scale, on the precise knowledge of the b quark fragmentation and on phenomenological models, e.g. the multiple interaction model. Based on the studies, a systematic uncertainty of less than 900MeV should be achievable.