Double Higgcision: 125 GeV Higgs boson and a potential diphoton Resonance

Searches for diphoton resonance have been shown to be very useful in discovering new heavy spin-0 or spin-2 particles. Supposing that a new heavy particle shows up in the diphoton channel and it points to a spin-0 boson, it can be allowed to have a small mixing with the observed 125 GeV Higgs-like boson. We borrow the example of the 750 GeV particles hinted with 3.2 fb$^{-1}$ data at the end of 2015 (though it did not appear in the 2016 data) to perform an analysis of "double Higgcision". In this work, we perform a complete Higgs-signal strength analysis in the Higgs-portal type framework, using all the existing 125 GeV Higgs boson data as well as the diphoton signal strength of the 750 GeV scalar boson. The best fit prefers a very tiny mixing between two scalar bosons, which has to be accommodated in models for the 750 GeV scalar boson.