Search for boosted diphoton resonances in the 10 to 70 GeV mass range using 138 $fb^{-1}$ of 13 TeV $pp$ collisions with the ATLAS detector

A search for diphoton resonances in the mass range between 10 to 70 GeV with the ATLAS experiment at the Large Hadron Collider is presented. The analysis is based on $pp$ collision data corresponding to an integrated luminosity of 138 $fb^{-1}$ at a centre-of-mass energy of 13 TeV recorded from 2015 to 2018. Previous searches for diphoton resonances have reached down to 65 GeV in mass, with no observed evidence for new physics. This search exploits the particular kinematics of events with close-by pairs of photons reconstructed in the detector, that allows to reach invariant masses down to 10 GeV, covering a previously unexplored region at hadron colliders due to the experimental challenges of recording low energy photons and estimating the backgrounds. No significant excess is observed and the reported limits provide the strongest bound on promptly-decaying axion-like-particles coupling to gluons and photons for masses between 10 and 70 GeV.