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.