Is symmetry breaking of S U ( 5 ) theory responsible for the diphoton excess
We advocate the possibility that the observed diphoton excess at 750 GeV at the LHC can be addressed by the scalar field that is a part of the SU(5) symmetry breaking sector. The field in question is the Standard Model singlet that resides in the adjoint representation that breaks SU(5) down to SU(3) x SU(2) x U(1). We also show that the required production and subsequent decay to two photons of this singlet can be induced by individual or combined contribution of two scalar multiplets $S_3$ and $R_2$ that transform as (3,3,-1/3) and (3,2,7/6) under SU(3) x SU(2) x U(1), respectively. The individual dominance of these multiplets is directly related to the issue of the charged fermion mass generation within the SU(5) framework and can be unambiguously tested through the diboson decay signatures of the Standard Model singlet field.