Fourth generationb′decays intob+Higgs boson

Abstract

If a fourth generation quark exists whose mass is below 255 GeV, then the only two-body charged current decay, ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}c+W,$ is doubly Cabibbo suppressed. For this reason, CDF has searched for the one-loop neutral current decay ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+Z,$ assuming that the branching ratio into $b+Z$ is 100%; an analysis giving the bounds on ${m}_{{b}^{\ensuremath{'}}}$ for smaller branching ratios is in preparation. In this Brief Report, we examine the neutral current decay ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+H,$ which will occur if the Higgs boson mass is less than ${m}_{{b}^{\ensuremath{'}}}\ensuremath{-}{m}_{b}.$ Four different cases are examined: the sequential case, the non-chiral isosinglet case, the non-chiral isodoublet case, and a two-Higgs-doublet model with flavor-changing neutral currents. In the first three of these, the rates for ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+Z$ and ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+H$ are comparable, assuming comparable phase space factors; in the fourth, ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+H$ is completely dominant. Thus, we emphasize the importance of giving ${b}^{\ensuremath{'}}$ mass bounds as a function of the branching ratio into $b+Z,$ since the assumption of a 100% branching ratio for ${b}^{\ensuremath{'}}\ensuremath{\rightarrow}b+Z$ may only be valid if the Higgs boson mass is near or above the ${b}^{\ensuremath{'}}$ mass.