Vectorlike fermions and Higgs couplings

New vectorlike fermions that mix with the third generation can significantly affect the τ and b Yukawa couplings. Consistent with precision electroweak measurements, the width of the Higgs boson to ττ, bb̄ 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 final states, such as γγ. New leptons can also substantially modify the Higgs boson branching ratio to photons through radiative effects, while new quarks can contribute to gg fusion. The combined effect can be as much as a factor of two on the branching ratio to γγ. The new quarks and leptons could be light, which would allow discovery at the LHC. In the case of significant suppression of h→ττ, searches for new leptons decaying to τ-rich final states, perhaps in association with Higgs bosons, are motivated.