Exclusion of an Exotic Top Quark with -4/3 Electric Charge Using Soft Lepton Tagging

We present a measurement of the electric charge of the top quark using p anti-p collisions corresponding to an integrated luminosity of 2.7 fb-1 at the CDF II detector. We reconstruct tt-bar events in the lepton + jets final state and use kinematic information to determine which b-jet is associated with the leptonically- or hadronically-decaying t-quark. Soft lepton taggers are used to determine the b-jet flavor. Along with the charge of the W boson decay lepton, this information permits the reconstruction of the top quark's electric charge. Out of 45 reconstructed events with 2.4 +/- 0.8 expected background events, 29 are reconstructed as tt-bar with the standard model +2/3 charge, whereas 16 are reconstructed as tt-bar with an exotic -4/3 charge. This is consistent with the standard model and excludes the exotic scenario at 95% confidence level. This is the strongest exclusion of the exotic charge scenario and the first to use soft leptons for this purpose.