Search for new heavy quarks in proton-antiproton collisions at $$\sqrt s = 0.63 TeV$$

We report on a search for new heavy quarks decaying semileptonically into muons, using 4.7 pb−1 of data taken during the CERN\(p\bar p\) collider runs in 1988 and 1989. The properties of isolated muons accompanied by jets are consistent with the predictions from the Standard Model but do not show a signal for a new heavy quark. Combining all the UA1 data (5.4 pb−1), lower mass limits are obtained at 60 GeV/c2 (95% CL) for thet-quark and 43 GeV/c2 (95% CL) for a fourth-generation charge-1/3 quark (b′-quark).