750 GeV diphoton excess and its explanation in two-Higgs-doublet models with a real inert scalar multiplet

We discuss a possible explanation of the recently observed diphoton excess at around 750 GeV as seen by the ATLAS and CMS experiments at the Large Hadron Collider. We calculate the cross section of the diphoton signature in two-Higgs-doublet models with the addition of a real isospin scalar multiplet without a vacuum expectation value, where a neutral component of such a representation can be a dark matter candidate. We find that the branching fraction of an additional CP-even Higgs boson H from the doublet fields into the diphoton mode can be significantly enhanced, by up to a factor of 10(3), with respect to the case of the simple two-Higgs-doublet model. Such a sizable enhancement is realized due to multicharged inert particle loops entering the H -> gamma gamma decay mode. Through this enhancement, we obtain a suitable cross section of the gg -> H -> gamma gamma process to explain the data with the fixed mass of H being 750 GeV.