Higgs precision study of the 750 GeV diphoton resonance and the 125 GeV standard model Higgs boson with Higgs-singlet mixing

We interpret the potential observation of the 750 GeV di-photon resonance at the LHC in models, in which an $SU(2)$ isospin-singlet scalar boson mixes with the Standard Model (SM) Higgs boson through an angle $\alpha$. Allowing the singlet scalar boson to have renormalizable couplings to vector-like leptons and quarks and introducing sizable decay width of the 750 GeV di-photon resonance into non-SM particles such as dark matters, we can explain the large production cross section $\sigma (H_2)\times B(H_2\to \gamma \gamma )$ as well as the apparent large total width of the boson without conflicts from the results obtained by previous global fits to the SM Higgs boson data.