Search for a fourth-generation charge -(1/3 quark

By studying $e^{+}$ $e^{\mathrm{-}}$ annihilations in the center-of-mass energy range between 50 and 60.8 GeV, we have established a 95%-confidence-level lower limit on the mass of a fourth-generation charge -(1/3 quark b’ of 27.2 GeV. In contrast with all previous searches, this limit has been obtained through consideration of the decay processes b’→bγ and b’→bg aswell as b’→${\mathrm{cW}}^{\mathrm{-}}$. For the cases where any one of the three decay modes dominates, we obtain higher mass limits.