  The ATLAS and CMS experiments at the LHC have reported an excess of diphoton
events with invariant mass around 750 GeV, with local significance of about
$3.6 \sigma$ and $2.6 \sigma$, respectively. We entertain the possibility that
this excess is due to new physics, in which case the data suggest a new
particle with 13 TeV LHC production cross section times diphoton branching
ratio of about 5~fb. Interestingly, ATLAS reports a mild preference for a
sizeable width for the signal of about 45 GeV; this result appears consistent
with CMS, and is further supported by improving the compatibility of the 8 TeV
and 13 TeV analyses. We focus on the possibility that the new state is a
scalar. First, we show that, in addition to the new state that is needed
directly to produce the diphoton bump, yet more new particles beyond the
Standard Model are needed to induce diphoton decay rate of the right size.
Second, we note that if the excess is attributed to the Breit-Wigner peak of a
single new state, then the signal strength and width -- taken together --
suggest a total LHC production cross section of order $10^5$ fb. Restricting to
perturbative models without ad-hoc introduction of many new states or exotic
charges, we reach the following conclusions: (i) Gluon-fusion cannot explain
the required large production cross section. (ii) Tree level production from
initial state quarks cannot explain the required branching ratio to two
photons. (iii) Tree level production is constrained by flavor data as well as
LHC Run-I and Tevatron dijet analyses. Insisting on a large width we are led to
suggest that more than one scalar states, nearly degenerate in mass, could
conspire to produce an observed wide bump.

High Energy Physics - Phenomenology