UC Santa Cruz

Results of a search for new phenomena in events with at least three photons are reported. Data from proton–proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb-1$$^{-1}$$, were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay Z→3γ$$Z\rightarrow 3\gamma $$, as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be σ×BR(h→aa)×BR(a→γγ)2<10-3σSM$$\sigma \times {\text{ BR }}(h \rightarrow aa) \times {\text{ BR }}(a \rightarrow \gamma \gamma )^{2} < 10^{-3} \sigma _{\text {SM}}$$ for 10 GeV $$m_{H} > $$ 125 GeV, and for a Z′$$Z'$$ decaying to three photons via Z′→a+γ→3γ$$Z' \rightarrow a+\gamma \rightarrow 3\gamma $$. Additionally, the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR(Z→3γ)<2.2×10-6$$(Z \rightarrow 3\gamma ) < 2.2 \times 10^{-6}$$, a result five times stronger than the previous result from LEP.