Vectorlike Fermions and Higgs Couplings

New vectorlike fermions that mix with the third generation can signi_cantly a_ect the_and b Yukawa couplings. Consistent with precision electroweak measurements, the width of the Higgs boson to_ _, b_b can be reduced by O(1) with respect to the Standard Model values. In the case of the b quark, a reduced width would result in an enhanced branching ratio for other_nal states, such as. New leptons can also substantially modify the Higgs boson branching ratio to photons through radiative e_ects, while new quarks can contribute to gg fusion. The combined e_ect can be as much as a factor of two on the branching ratio to. The new quarks and leptons could be light, which would allow discovery at the LHC. In the case of signi_cant suppression of h!_ _, searches for new leptons decaying to _-rich _nal states, perhaps in association with Higgs bosons, are motivated