Search for anomalous couplings in top decay at hadron colliders

We present a quantitative study on sensitivities to the top-decay anomalous couplings, taking into account realistic experimental conditions expected at Tevatron and LHC. A double angular distribution of W and charged lepton in the top-decay is analyzed, using t events in the leptons+jets channel. In order to improve sensitivities to the anomalous couplings, we apply two techniques: (i) We use a likelihood fitting method for full kinematical reconstruction of each top event. (ii) We develop a new effective spin reconstruction method for leptonically-decayed top quarks; this method does not require spin information of the antitop side. For simplicity, we neglect couplings of right-handed bottom quark as well as CP violating couplings. The 95% C.L. estimated bound on a ratio of anomalous couplings reads -0.81 < f(2)/f(1)<-0.70, -0.12 < f(2)/f(1)< 0.14 using 1000 reconstructed top events at Tevatron, while -0.74 < f(2)/f(1)<-0.72, -0.01 < f(2)/f(1)< 0.01 is expected with 100 k reconstructed top events at LHC, where only statistical errors are taken into account. A two-fold ambiguity in the allowed range remains when the number of events exceeds a few hundred.