Search for diphoton resonances in the 66 to 110 GeV mass range using 140 fb$^{-1}$ of 13 TeV $pp$ collisions collected with the ATLAS detector

Many extensions to the Higgs sector of the Standard Model motivate additional spin-0 bosons. This search focuses on light, spin-0 bosons decaying to two photons in the 66 to 110 GeV mass range, using 140 fb$^{-1}$ of $pp$ collisions produced by the Large Hadron Collider and collected by the ATLAS detector. Multi-variate analysis techniques are used to both mitigate background processes and classify events, improving the sensitivity to physics beyond the Standard Model. Additionally, a Gaussian Process regression is used to maintain a non-dominant uncertainty on the background model. Both a model-independent search for a generic spin-0 particle and a model-dependent search for an additional low-mass Higgs boson are performed. No significant excess is observed, and an upper limit at the 95% CL is set on total (fiducial) cross-section times branching ratio from 8 to 53 fb (19 to 102 fb) for the model-independent (model-dependent) result.