  A search for diphoton resonances in the mass range between 10 and 70 GeV with
the ATLAS experiment at the Large Hadron Collider (LHC) 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 at the LHC have
explored masses down to 65 GeV, finding no evidence of new particles. This
search exploits the particular kinematics of events with pairs of closely
spaced photons reconstructed in the detector, allowing examination of invariant
masses down to 10 GeV. The presented strategy covers a region previously
unexplored at hadron colliders because of 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.
