Vectorlike Fermions and Higgs Couplings
  New vectorlike fermions that mix with the third generation can significantly affect the tau and b Yukawa couplings. Consistent with precision electroweak measurements, the width of the Higgs boson to tau tau, 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 final states, such as gamma gamma. New leptons can also substantially modify the Higgs boson branching ratio to photons through radiative effects, while new quarks can contribute to g g fusion. The combined effect can be as much as a factor of two on the branching ratio to gamma gamma. The new quarks and leptons could be light, which would allow discovery at the LHC. In the case of significant suppression of h -> tau tau, searches for new leptons decaying to tau-rich final states, perhaps in association with Higgs bosons, are motivated. 
John Kearney, Aaron Pierce and Neal Weiner
hep-ph