A search for pairs of highly collimated groupings of photons in pp collisions at 13 TeV with the ATLAS detector

A search for a new scalar resonance decaying to a pair of photon-jets—collimated groupings of photons—is performed with the ATLAS detector at the Large Hadron Collider. Data from proton–proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.7 /fb, were collected in 2015 and 2016. Highly collimated 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. No statistically significant excess of events from the background expectation is observed, and 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).