Probing eμ flavor-violating ALP at Belle II

Recently, it was pointed out that the electron and muon g - 2 discrepancies can be explained simultaneously by a single flavor-violating axion-like particle (ALP). We show that the parameter regions favored by the muon g - 2 are already excluded by the muonium-antimuonium oscillation bound. In contrast, those for the electron g - 2 can be consistent with this bound when the ALP is heavier than 1.5 GeV. We propose to search for a signature of the same-sign and same-flavor lepton pairs and the forward-backward muon asymmetry to test the model at the Belle II experiment.