Search for axion-like particles decaying to two photons at Belle II

Axion-like particles (ALPs) are predicted in many extensions of the Standard Model and provide a well-motivated portal between visible and hidden sectors through their coupling to photons. We search for ALPs produced in the process $e^{+}{e}^{-}\to \gamma a$, $a\to \gamma \gamma$, using a data sample corresponding to an integrated luminosity of $408{\mathrm{f}\mathrm{b}}^{-1}$ recorded by the Belle~II detector at the SuperKEKB $e^{+}{e}^{-}$ collider. Events containing three photons are used to reconstruct the ALP as a narrow peak in the di-photon invariant mass spectrum over the range $0.17<m_a<9.80\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$. No significant excess above background is observed. We set 95% confidence level upper limits on the production cross section and on the ALP-photon coupling $g_{a\gamma \gamma }$, reaching sensitivities at the level of ${10}^{-4}{\mathrm{G}\mathrm{e}\mathrm{V}}^{-1}$. The limits are the most restrictive to date over nearly the entire mass range $0.17<m_a<5.00\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$, and improve upon previous results by up to a factor 9.