Diphotons from tetraphotons in the decay of a 125 GeV Higgs boson at the LHC

Recently the ATLAS and CMS experiments have presented data hinting at the presence of a Higgs boson at m _h≃125GeV. The best-fit h→γγ rate averaged over the two experiments is approximately 2.1±0.5 times the standard model prediction. We study the possibility that the excess relative to the standard model is due to h→aa decays, where a is a light pseudoscalar that decays predominantly into γγ. Although this process yields 4γ final states, if the pseudoscalar has a mass of the order tens of MeV, the two photons from each a decay can be so highly collimated that they may be identified as a single photon. Some fraction of the events then contribute to an effective h→γγ signal. We study the constraints on the parameter space where the net h→γγ rate is enhanced over the standard model by this mechanism and describe some simple models that give rise to the pseudoscalar-photon interaction. Further tests and prospects for searches in the near future are discussed.