Light scalar top quark at e/sup +/e/sup -/ colliders

We examine the production and decay of a supersymmetric scalar top t-italic-tilde/sub 1/ at e/sup +/e/sup -/ 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 t-italic-tilde/sub 1/ 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 t-italic-tilde/sub 1/ decay modes in this scenario. The flavor-changing decay t-italic-tilde/sub 1/..-->..cgamma-tilde is found to proceed through the misalignment of the scalar-quark and quark mixing matrices. If the three-body decay t-italic-tilde/sub 1/..-->..bl/sup +/nu-tilde is kinematically accessible it dominates the two-body decay. The decay t-italic-tilde/sub 1/..-->..b..nu..l-tilde/sup +/ 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 t-italic-tilde/sub 1/ pair production cross section including the ..gamma.. and Z contribution and the left-right mixing is presented.