Decoupling of anomalous top-quark-decay vertices in angular distribution of secondary particles

Angular distribution of a secondary particle from top-quark decays is studied in a simple and general manner, paying careful attention to how relevant the top-quark production mechanism is. The conditions that lead to the distribution free from any possible anomalous top-quark-decay interactions are specified. Some approximations adopted in earlier papers are relaxed and their relevance is discussed.