Diphotons at the Z-pole in models of the 750 GeV resonance decaying to axion-like particles

Models in which the 750 GeV resonance (S) decays to two light axion-like particles (ALPs a), which in turn decay to collimated photons mimicking the observed signal, are motivated by Hidden Valley scenarios and could also provide a mechanism by which a S → γγ signal persists while S → Zγ, ZZ and W W remain subdued in the near future. We point out that these Hidden Valley like models invoking S → aa → 4γ must also contend with Z → a(→ γγ)γ constraints coming from CDF and ATLAS. Within an effective field theory framework, we work out the constraints on the couplings of S to a and gauge bosons coming from photonic Z decays and ensuring that the ALPs decay inside the electromagnetic calorimeter, 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 a large as well as a narrow width, and for different relative contributions to the signal coming from S → γγ and a → γγ. Results for the particular case where S and a belong to the same complex field are also presented. A γγ resonance at the Z-pole coming from Z → aγ is expected in this class of models. Taking benchmark ALP masses below around 0.4 GeV and, assuming reasonable values for the fake jet rate and the identification efficiency of the photon-jet, we find the prospects for the discovery of diphotons at the Z-pole.