Search for a fourth generation charge -1/3 quark via flavor changing neutral currents

There is some likelihood that a light (< mt) fourth generation charge -1/3 quark (b') would decay predominantly via loop induced flavor changing neutral currents. The charged current decay of b' to charm would be highly Cabibbo suppressed due to the fact that it changes the generation number by two. The D0 experiment has searched for b' pair production where one or both b' quarks decays via b' → b+γ, giving signatures photon + three jets and two photons + two jets. WE don not see a significant excess of such events over background. In both modes, we set an upper limit on the cross section times branching ratio that is sufficient to rule out a standard sequential b' decaying predominantly via FCNC in the mass range mZ/2 < mb' < mZ + mb. For b' masses larger than this, the dominant FCNC decay mode is expected to be b' → b + Z. 14 refs., 13 figs., 5 tabs