Fusing photons into diphoton resonances at Belle II and beyond

We propose a new search for a diphoton resonance in the e$^+$e$^−$ + γγ final state at Belle II where the full final state is reconstructed in the detector. This new channel improves the expected reach compared to the γ + γγ channel in most of the available mass range. For simplicity we show our results in the simple parameter space of an ALP coupled solely to Standard Model photons. In addition, we show how an extension of the forward coverage of Belle II, or another similar experiment at the high intensity frontier, could improve the reach in our channel. We show that such a forward extension can be advantageous even with a loss of a factor 100 in luminosity compared to Belle II.