A light top quark after all?

A massive charged scalar, such as is present in a two Higgs-doublet electroweak theory, can alter the nature of top-quark decay, ensuring that the top quark would not have been detected at hadron colliders. Box diagrams involving charged-scalar exchange can produce the observed level of B0− B 0 mixing, thus relaxing the need for a heavy top quark. Future experiments can readily establish the existence of the charged scalar.