Confronting the fourth generation two-Higgs-doublet model with the phenomenology of heavy Higgs bosons

A sequential fourth generation is known to be excluded because the nondecoupling contribution to κg, the Higgs coupling modifier with a gluon pair, is unacceptably large. Recently a new way to save the model was suggested in the type-II two-Higgs-doublet model: If the Yukawa couplings of down-type fermions have the wrong sign, the contributions from t′ and b′ to κg are canceled. We study the theoretical and experimental constraints on this model, focusing on the heavy Higgs bosons. We point out two constraining features. First, the exact wrong-sign limit does not allow alignment, which causes the perturbative unitarity for the scalar-scalar scattering to put upper bounds on the heavy Higgs boson masses like MH, MA≲920 GeV and MH±≲620 GeV. Second, the Yukawa couplings of the fourth generation fermions to the heavy Higgs bosons are generically large, being proportional to the heavy fermion mass and, for the down-type fermions, to tanβ as well. The gluon fusion production of H and A through the fourth generation quark loops becomes significant. We find that the current LHC data on pp→ZZ for H, along with the theoretical and indirect constraints, exclude the model at leading order.