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Models in which the 750 GeV resonance ($S$) decays to two light axion-likeparticles (ALPs $a$), which in turn decay to collimated photons mimicking theobserved signal, are motivated by Hidden Valley scenarios and could alsoprovide a mechanism by which a $S \to \gamma \gamma$ signal persists while $S\to Z \gamma,\; ZZ$ and $WW$ remain subdued in the near future. We point outthat these Hidden Valley like models invoking $S \to aa \to 4 \gamma$ must alsocontend with $Z \to a (\to \gamma \gamma) \gamma$ constraints coming from CDFand ATLAS. Within an effective field theory framework, we work out theconstraints on the couplings of $S$ to $a$ and gauge bosons coming fromphotonic $Z$ decays and ensuring that the ALPs decay inside the electromagneticcalorimeter, in two regimes - where $a$ decays primarily to photons, and where$a$ also has hadronic branchings. The analysis is done for both when $S$ has alarge as well as a narrow width, and for different relative contributions tothe signal coming from $S \to \gamma \gamma$ and $a \to \gamma \gamma$. Resultsfor the particular case where $S$ and $a$ belong to the same complex field arealso presented. A $\gamma\gamma$ resonance at the $Z$-pole coming from $Z \to a\gamma$ is expected in this class of models. Taking benchmark ALP masses belowaround 0.4 GeV and, assuming reasonable values for the fake jet rate and theidentification efficiency of the photon-jet, we find the prospects for thediscovery of diphotons at the $Z$-pole.

Diphotons at the $Z$-pole in Models of the 750 GeV Resonance Decaying to Axion-Like Particles