We consider models for the di-photon resonance observed at ATLAS (with $3.6{\mathrm{fb}}^{-1}$) and CMS (with $2.6{\mathrm{fb}}^{-1}$). We find there is no conflict between the signal reported at 13 TeV and the constraints from both experiments at 8 TeV with $20.3{\mathrm{fb}}^{-1}$. We make a simple argument for why the decay to the $\gamma \gamma$ mode must be generated by additional, beyond the standard model (SM) states. We explore four viable options: (i) resonance production and decay through loops of messenger fermions or scalars, (ii) a resonant messenger which decays to the $\text{di-photon resonance}+\mathrm{X}$, (iii) an edge configuration where $A\to B\gamma \to C\gamma \gamma$, and (iv) Hidden Valley-like models where the resonance decays to a pair of very light (sub-GeV) states, each of which in turn decays to a pair of collimated photons that cannot be distinguished from a single photon. Since in each case multiple new states have been introduced, a wealth of signatures is expected to ensue at run 2 of the LHC.

• Received 12 February 2016

DOI:https://doi.org/10.1103/PhysRevD.93.075020