We report the most sensitive direct search for pair production of fourth-generation bottomlike chiral quarks ($b^{\prime }$) each decaying promptly to $tW$. We search for an excess of events with an electron or muon, at least five jets (one identified as due to a $b$ or $c$ quark), and an imbalance of transverse momentum by using data from $p\overline{p}$ collisions collected by the CDF II detector at Fermilab with an integrated luminosity of $4.8{\mathrm{fb}}^{-1}$. We observe events consistent with background expectation, calculate upper limits on the $b^{\prime }$ pair-production cross section (${\sigma }_{b{\overline{b}}^{\prime }}\lesssim 30\mathrm{fb}$ for $m_{b^{\prime }}>375\mathrm{GeV}/c^2$), and exclude $m_{b^{\prime }}<372\mathrm{GeV}/c^2$ at 95% confidence level assuming a 100% branching ratio of $b^{\prime }$ to $tW$.

• Received 29 January 2011

DOI:https://doi.org/10.1103/PhysRevLett.106.141803