  We investigate diboson signals in the Standard Model (SM) with an extended
Higgs sector, motivated by the excesses in the diboson channels at the LHC. We
begin with the unitarity sum-rules of the weak gauge boson scattering assuming
the Higgs sector is extended. According to the sum-rules, we discuss the Higgs
interpretations of the diboson signals and the consistency with the ATLAS
diboson anomaly and other experimental constraints. As a concrete example, we
propose a two-Higgs-doublet model where the Yukawa coupling of an extra CP-even
scalar with up-type quark is relatively large. The diboson (WW and ZZ) signals
can be explained by 2 TeV CP-even Higgs boson, while the partners, the CP-odd
and the charged Higgs bosons in the extra doublet, are severely constrained by
both the LHC direct search and the indirect search via flavor observables.
Especially, in order to avoid the diboson resonance search in the Vh channel,
further extensions of the model are required. The diboson excess is correlated
with the SM Higgs signals in our framework, so that the precise measurement of
the SM Higgs boson is also important to test the Higgs interpretation.
