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

Abstract

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.

ALPs effective field theory and collider signatures

“…Nevertheless, ALPs may well show up first at colliders [35][36][37] or in rare mesonic decays [38,39], and the SU (2) L × U (1) Y invariant formulation of their interactions developed here provides new beautiful channels involving the electroweak gauge bosons and the Higgs particle. In the second -phenomenological -part of this work, the foreseen impact of those couplings at colliders and in particular at LHC will be analyzed for the first time, identifying the new signals and performing a detailed analysis of experimental bounds and prospects.…”

Dark photon relic dark matter production through the dark axion portal

“…Though we have limited ourselves to the conventional QCD axion case in this paper where m a ∼ (10 −5 − 10 −2 ) eV and G aγγ ∼ (10 −11 − 10 −14 ) GeV −1 , our discussion can be extended to rather a wide class of models with an axion-like particle (ALP) whose mass and coupling can be significantly larger than the QCD axion. In such models, various types of experiments can be used to test the dark axion portal, perhaps in a similar way that the colliders give constraints on the axion-photon-Z boson coupling by measuring Z → 3γ following Z → γ + ALP for a MeV-GeV scale ALP [66,67]. For instance, a mono-photon signal from Z → γ + 2γ followed by Z → γ + ALP and gg → ALP → γγ would be possible.…”

Precision tools and models to narrow in on the 750 GeV diphoton resonance

“…The parameters are easily exchanged between MicrOmegas and a SARAH-based spectrum generator by copying the spectrum file into the main directory of the current MicrOmegas project directory. 17 However, it is important to remember that MicrOmegas cannot handle complex parameters. Therefore, one has to make sure, even in the case without CP violation, that all rotation matrices of Majorana fermions are real.…”