Search for pairs of highly collimated groupings of photons at 13 TeV with the ATLAS detector

A search for a new scalar resonance particle decaying to a pair of photon-jets—collimated groupings of photons—is performed with the ATLAS detector at the Large Hadron Collider. The dataset of pp collisions at a centre-of-mass energy of 13 TeV was collected in 2015 and 2016, corresponding to an intergrated luminosity of 36.7 /fb. Photon-jets arise when a high-mass scalar particle decays into a pair of light resonance particles, which consecutively decay to photons. When these light particles are highly boosted, their decay to photons lead to photon-jets. Candidate events with a pair of photon-jets are searched for in events containing two reconstructed photons with high transverse energy. No statistically significant excess of events from the background expectation is observed. The results are interpreted in the scenario of a scalar particle with a narrow width (X) decaying into a pair of spin-0 particles (a). Upper limits are placed on the product of the cross-section and branching ratios σ*B(X->aa)*B(a->γγ)^2 and σ*B(X->aa)*B(a->3π0)^2 for the ranges 200 GeV < m_X < 2 TeV and m_a < 0.01*m_X.