Search for new phenomena in events with at least three photons collected in pp collisions at √s = 8 TeV with the ATLAS detector

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, 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, 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 < 10⁻³ σ_SM for 10 GeV < m_a <  62 GeV. Limits are also presented for Higgs boson-like scalars (H) for m_H > 125 GeV, and for a Z′ decaying to three photons via Z′ → a + γ → 3γ. 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, a result five times stronger than the previous result from LEP.