Recently, the possible new signature of 750GeV Diphoton is widely discussed.
In this talk, I discuss the possibility to explain the the signal by the bound state of heavy fermion X.
We found that the best-fit signal cross section is obtained for the SU(2)L singlet colored fermion X with YX=4/3. Having such an exotic hypercharge, the particle is expected to decay through some higher dimensional operators, consistent with the small width assumption. The decay of X may involve a stable particle χ, if both X and χ are odd under some conserved Z2 symmetry. In that case, the particle X suffers from the constraints of jets + missing ET searches by ATLAS and CMS at 8 TeV and 13 TeV. We found that such a scenario still survives if the mass difference between X and χ is above ∼ 30 GeV for mX∼375 GeV. Even assuming pair annihilation of χ is small, the relic density of χ is small enough if the mass difference between X and χ is smaller than ∼40 GeV.