Search for a Fourth-Generation Quark More Massive than the Z0 Boson in ppbar Collisions at sqrt(s) = 1.8 TeV

TL;DR

This paper reports a search for a hypothetical fourth-generation b' quark in proton-antiproton collisions at 1.8 TeV, setting mass exclusion limits based on decay signatures involving Z bosons, leptons, and jets.

Contribution

The study provides the first experimental mass limits for a fourth-generation b' quark decaying via flavor-changing neutral currents at Tevatron energies.

Findings

Excluded b' quark masses between 100 and 199 GeV/c^2 at 95% confidence level for BR(b' -> bZ) = 100%.

Set upper limits on the production cross section times branching ratio squared as a function of b' mass.

Analyzed decay signatures involving leptons, jets, and Z bosons to search for the b' quark.

Abstract

We present the results of a search for pair production of a fourth-generation charge -1/3 quark (b') in sqrt(s)=1.8 TeV ppbar 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' -> bZ and that the b' mass is greater than m_Z + m_b. We studied the decay mode b'b'bar -> ZZ b bbar where one Z0 decays into e^+e^- or mu^+ mu^- and the other decays hadronically, giving a signature of two leptons plus jets. An upper limit on the cross section of ppbar -> b'b'bar times [BR (b' -> bZ)]^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/c^2 for BR(b' -> bZ) = 100%.