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

Abstract

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 $e^{+}{e}^{-}\to {\hat{I}}^{3}a$, $a\to {\hat{I}}^3{\hat{I}}^3$, using a data sample corresponding to an integrated luminosity of $408{\mathrm{fb}}^{-1}$ 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 $0.17<m_a<9.80\mathrm{GeV}/c^2$. 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 $g_{a{\hat{I}}^3{\hat{I}}^3}$, reaching sensitivities at the level of $10^{-4}{\mathrm{GeV}}^{-1}$. The limits are the most restrictive to date over nearly the entire mass range $0.17<m_a<5.00\mathrm{GeV}/c^2$, and improve upon previous results by up to a factor 9.

9 pages, 4 figures, Submitted for publication to Physical Review Letters