Higgs phenomenology in Type-I 2HDM with U(1)H Higgs gauge symmetry

It is well known that generic two-Higgs-doublet models (2HDMs) suffer from potentially large Higgs-mediated flavor-changing neutral current (FCNC) problem, unless additional symmetries are imposed on the Higgs fields thereby respecting the Natural Flavor Conservation Criterion (NFC) by Glashow and Weinberg. A common way to respect the NFC is to impose Z2 symmetry which is softly broken by a dim-2 operator. Another new way is to introduce local U(1)H Higgs flavor symmetry that distinguishes one Higgs doublet from the other. In this paper, we consider the Higgs phenomenology in Type-I 2HDMs with the U(1)H symmetry with the simplest U(1)H assignments that the SM fermions are all neutral under U(1)H , and we make detailed comparison with the ordinary Type-I 2HDM. After imposing various constraints such as vacuum stability and perturbativity as well as the electroweak precision observables and collider search bounds on charged Higgs boson, we find that the allowed Higgs signal strengths in our model are much broader than those in the ordinary Type-I 2HDM, because of newly introduced U(1)H-charged singlet scalar and U(1)H gauge boson. Still the ATLAS data on gg → h → γγ cannot be accommodated. Our model could be distinguished from the ordinary 2HDM with the Z2 symmetry in a certain parameter region and some channels. If the couplings of the new boson turn out to be close to those in the SM, it would be essential to search for extra U(1)H gauge boson and/or one more neutral scalar boson to distinguish two models.