Search for a fourth-generation quark more massive than the Z(0) boson in proton-antiproton collisions at sqrt(s) = 1.8 TeV.

摘要

The Standard Model with three generations of quarks and leptons is in excellent agreement with all experimental data available today. However, the Standard Model does not explain either the fermion family replication or the fermion mass hierarchy. Several theoretical models have been proposed to solve shortcomings in the Standard Model through the introduction of extra quarks and leptons, while grand unification, supersymmetry, supergravity and superstrings predict or can accomodate extra fermion states.;This dissertation presents 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 the pair-produced b' quarks decay via the flavor-changing neutral current process b'→bZ0 and that the b' mass is greater than mZ + mb. We studied the decay mode b'b'→b bZ0Z0 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 spp→b'b 'xBRb '→bZ0 2 is established as a function of the b' mass. A b' quark with mass between 100 and 199 GeV/c2 is excluded at 95% confidence level, assuming that BRb'→bZ0 =100%. .