Search for a fourth-generation quark more massive than the Z₀ boson in pp^¯ collisions at √s = 1.8 TeV

We present the results of a search for pair production of a fourth-generation charge -1/3 quark (b′) in √s=1.8 TeV pp^¯ collisions using 88 pb⁻¹ of data obtained with the Collider Detector at Fermilab. We assume that both quarks decay via the flavor-changing neutral current process b′→bZ⁰ and that the b′ mass is greater than m_Z+m_b. We studied the decay mode b′b′^¯→Z⁰Z⁰bb^¯ where one Z⁰ decays into e⁺e⁻ or μ⁺μ⁻ and the other decays hadronically, giving a signature of two leptons plus jets. An upper limit on the σ_{pp^¯→b′b′^¯}×[B(b′→bZ⁰)]² is established as a function of the b′ mass. We exclude at 95% confidence level a b′ quark with mass between 100 and 199 GeV/c² for B(b′→bZ⁰)=100%.