Axion-mediated dark matter and Higgs diphoton signal

We consider axion-mediated dark matter models motivated by Fermi gamma ray line at 130 GeV, where an axion-like scalar couples to a singlet Dirac fermion dark matter (DM) so its anomaly interactions with electroweak gauge bosons allow the DM to annihilate into monochromatic photon(s). In these models, extra vector-like leptons generate the necessary anomaly interactions for the axion and can also modify the Higgs-to-diphoton rate by their Yukawa couplings to the SM Higgs boson. We can distinguish models by the branching fraction of the DM annihilation into a photon pair, favoring the model with a triplet fermion. From the condition that the lighter charged extra lepton must be heavier than dark matter for no tree-level DM annihilations, we also show that the ratio of Higgs-to-diphoton rate to the SM value is constrained by vacuum stability to 1.4(1.5) for the cutoff scale of 10(1) TeV.