Electron and muon reconstruction in single leptonic ttbar events

The CMS detector is designed to observe electrons and muons in a wide range of energies. Therefore in a well populated event, several leptons could be reconstructed and their flavour identified. In the reconstruction of single-lepton ttbar events it is important to identify the leptons which are originating directly from the decay of the W boson and separate them from those which are produced in the fragmentation process of the partons. This note describes a likelihood ratio method which separates both categories for the lepton being either an electron or a muon. The likelihood ratio method is based on the observation that some variables can differentiate between both categories, namely the isolation energy or isolation transverse momentum, the isolation angle and the transverse momentum of the reconstructed lepton. Also reconstructed vertex information is used on the basis of the significance of the distance in the direction along the beam line between lepton and primary vertex. The efficiencies are compared with those obtained by the HLT lepton isolation criteria. The result for muons is found to be competitive, while for electrons a significant improvement is observed. The influence on the efficiency from pile-up has been studied and found to be marginal.