Light scalar top quark at e+e- colliders

We examine the production and decay of a supersymmetric scalar top t1 at e+e- colliders such as TRISTAN at KEK, the Stanford Linear Collider, and LEP at CERN, based on supersymmetric models with soft breaking. Existing bounds for supersymmetric particles imply that t1 in the energy range of these colliders would be lighter than other scalar quarks and than the top quark. We make a careful study of available t1 decay modes in this scenario. The flavor-changing decay t1c is found to proceed through the misalignment of the scalar-quark and quark mixing matrices. If the three-body decay t1bl+ is kinematically accessible it dominates the two-body decay. The decay t1bl+ is even larger due to different chirality structure, but is likely to suffer from a large phase-space suppression. Four-body decays are negligible. The t1 pair production cross section including the and Z contribution and the left-right mixing is presented.