Vector Quarks in the Higgs Triplet Model
  We analyze the effects of introducing vector fermions in the Higgs Triplet Model. In this scenario, the model contains, in addition to the Standard Model particle content, one triplet Higgs representation, and a variety of vector-like fermion states, including singlet, doublet, and triplet states. We investigate the electroweak precision variables and impose restrictions on model parameters. We show that, for some representations, introducing vector quarks significantly alters the constraints on the mass of the doubly charged Higgs boson, bringing it in closer agreement with experimental constraints. We also study the effects of introducing the vector-like fermions on neutral Higgs phenomenology, in particular on the loop-dominated decays H -> gamma gamma and H -> Z gamma, and the restrictions they impose on the parameter space. 
Sahar Bahrami and Mariana Frank
hep-ph