Search for a fourth-generation quark with -Q-=e/3 in e+e- collisions at s =56"57 GeV

A search for a fourth-generation quark with \ensuremath{\Vert}Q\ensuremath{\Vert}=e/3 has been made with the VENUS detector at the KEK ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ collider TRISTAN. Multihadron events with a spherical shape or containing isolated leptons were studied. There is no evidence for an excess production of such events in ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ collision at \ensuremath{\surd}s =56--57 GeV and a lower limit on the mass is 27.5 GeV/${c}^{2}$ at the 95% C.L.