On the validity of the CMSSM interpretation of the diphoton excess

It has been proposed that the observed diphoton excess at 750 GeV could be explained within the constrained MSSM via resonantly produced stop bound states. We reanalyse this scenario critically and extend previous work to include the constraints from the stability of the electroweak vacuum and from the decays of the stoponium into a pair of Higgs bosons. It is shown that the interesting regions of parameter space with a light stop and Higgs of the desired mass are ruled out by these constraints. This conclusion isn't affected by the presence of the bound states because the binding energy is usually very small in the regions of parameter space which can explain the Higgs mass. Thus, this also leads to strong constraints on the diphoton production cross section which is in general too small.