Radiative decays of a singlet scalar boson through vectorlike quarks

If the standard model Higgs boson were much heavier, it would appear as a broad resonance since its decay into a pair of longitudinally polarized gauge bosons is highly enhanced. We study whether the same enhancement happens at loop level in a simple extension of the standard model with a singlet scalar boson S and three vectorlike quark multiplets. In order to focus on the loop effects, we assume that S does not interact with the standard model particles at tree level. Vectorlike quarks running in the loop link the singlet scalar S to the standard model world. There are two kinds of loop effects in the S phenomenology-the mixing with the Higgs boson and the radiative decays into hh, WW, ZZ, gg, and γγ. We show that the crucial conditions for the loop-induced longitudinal polarization enhancement are the large mass differences among vectorlike quarks. The current LHC constraints from the heavy scalar searches and the Higgs precision data are shown to be very significant: The mixing angle with the Higgs boson should be smaller than about 0.1 for mS=750 GeV.