Search for a Fourth Generation Charge $-1\mathrm{}/3$ Quark via Flavor Changing Neutral Current Decay

We report on a search for pair production of a fourth generation charge $-1\mathrm{}/3$ quark ( $b^{\prime }$) in $p\overline{p}$ collisions at $\sqrt{s}=1.8\mathrm{}$ TeV by the DØ experiment at the Fermilab Tevatron using an integrated luminosity of 93 ${\mathrm{pb}}^{-1\mathrm{}}$. Both $b^{\prime }$ quarks are assumed to decay via flavor changing neutral currents (FCNC). The search uses the signatures $\gamma +3\mathrm{}$ jets $+\mu$-tag and $2\gamma +2\mathrm{}$ jets. We see no significant excess of events over the expected background. We place an upper limit on the production cross section times branching fraction that is well below theoretical expectations for a $b^{\prime }$ decaying exclusively via FCNC for $b^{\prime }$ masses up to $m_Z{+m}_b$.