Search for isolated photons from flavor-changing neutral-current decay of a new quark at the KEK e+e- collider TRISTAN

A search for a new charge {minus} 1/3 quark has been carried out at the KEK {ital e}{sup +}{ital e}{sup {minus}}Collider TRISTAN under the assumption of its photonic decay through a flavor-changing neutral current (FCNC). The observed number of multihadronic events with isolated photons is consistent with that expected from the known five quark flavors. Including the usual charged-current decays, limits on the absolute photonic branching ratio and FCNC processes were obtained.