Search for new heavy quarks in electron-muon events at the Fermilab Tevatron Collider

A search for tt¯→eμ+X in pp¯ collisions at s√¯ TeV is described. The production and decay of top-quark–antiquark pairs is considered in the context of the standard model. The analysis is based on data with an integrated luminosity of 4.4 ${\mathrm{pb}}^{\mathrm{-}1}$ recorded with the Collider Detector at Fermilab. An upper limit on the tt¯ cross section is obtained, and the top quark in the mass range 28–72 GeV/$c^2$ is excluded at the 95% C.L. The same limits apply to a possible fourth-generation, charge -(1/3, b’ quark, decaying via the charged current.