High pt Top Mass Reconstruction in the Single Lepton Plus Jets Channel Using a Large Calorimeter Cluster

A new very promising method for the top mass reconstruction in ATLAS is presented. The method uses a special sub-sample of the single lepton plus jet events where the top has high pt (pt>200 GeV), available with acceptable statistics at LHC due to the high ttbar production rate. The top mass is reconstructed as the invariant mass of all the calorimeter towers in a cone around the top quark direction. From studies made with both the fast and full detector simulation programs, it is shown that an accuracy of 1-1.5\% in the reconstructed top quark mass can be achieved, using only one year's data at low luminosity of LHC (10 fb^{-1}). A detailed study of the systematic errors involved with the measurement is discussed as well.