Search for a Fourth-Generation Quark More Massive than theZ0Boson inpp¯Collisions at√s=1.8TeV

We present the results of a search for pair production of a fourth-generation charge $-\frac{1}{3}$ quark $\left(b^{\prime }\right)$ in $\sqrt{s}=1.8\mathrm{}\mathrm{TeV}$ $p\overline{p}$ collisions using $88{\mathrm{pb}}^{-1\mathrm{}}$ of data obtained with the Collider Detector at Fermilab. We assume that both quarks decay via the flavor-changing neutral current process $b^{\prime }\to {\mathrm{bZ}}^0$ and that the $b^{\prime }$ mass is greater than $m_Z{+m}_b$. We studied the decay mode $b^{\prime }\overline{b^{\prime }}\to Z^0{Z}^{0}b\overline{b}$ where one $Z^0$ decays into $e^{+}{e}^{-}$ or ${\mu }^{+}{\mu }^{-}$ and the other decays hadronically, giving a signature of two leptons plus jets. An upper limit on the ${\sigma }_{p\overline{p}\to b^{\prime }\overline{b^{\prime }}}\times \left[B\right(b^{\prime }\to {\mathrm{bZ}}^0){]}^2$ is established as a function of the $b^{\prime }$ mass. We exclude at $95\%$ confidence level a $b^{\prime }$ quark with mass between 100 and $199\mathrm{GeV}{/c}^2$ for $B(b^{\prime }\to {\mathrm{bZ}}^0)=100\mathrm{}\%$.