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

Abstract

By studying ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ 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'\ensuremath{\rightarrow}b\ensuremath{\gamma} and b'\ensuremath{\rightarrow}bg aswell as b'\ensuremath{\rightarrow}${\mathrm{cW}}^{\mathrm{\ensuremath{-}}}$. For the cases where any one of the three decay modes dominates, we obtain higher mass limits.