  We consider the diphoton excess observed by ATLAS and CMS using the most
up-to-date data and estimate the preferred enhancement in the production rate
between 8 TeV and 13 TeV. Within the framework of effective field theory (EFT),
we then show that for both spin-0 and spin-2 Standard Model (SM) gauge-singlet
resonances, two of the three processes S to ZZ, S to Z gamma, and S to WW must
occur with a non-zero rate. Moreover, we demonstrate that these branching
ratios are highly correlated in the EFT. Couplings of S to additional SM states
may be constrained and differentiated by comparing the S production rates with
and without the vector-boson fusion (VBF) cuts. We find that for a given VBF to
inclusive production ratio there is maximum rate of S to gauge bosons, b b-bar,
and lighter quark anti-quark pairs. Simultaneous measurements of the width and
the VBF ratio may be able to point towards the existence of hidden decays.
