Search for Axion-Like Particles produced in $e^{+}{e}^{-}$ collisions at Belle II

Abstract

We present a search for the direct production of a light pseudoscalar $a$ decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process $e^{+}{e}^{-}\to {\hat{I}}^{3}a,a\to {\hat{I}}^3{\hat{I}}^3$ in the mass range ${0.2} \,< m_a < {9.7}\,{\text{GeV/$c$}^2}$ using data corresponding to an integrated luminosity of $(445\pm 3){\text{pb}}^{-1}$. Light pseudoscalars interacting predominantly with standard model gauge bosons (so-called axion-like particles or ALPs) are frequently postulated in extensions of the standard model. We find no evidence for ALPs and set 95% confidence level upper limits on the coupling strength $g_{a{\hat{I}}^3{\hat{I}}^3}$ of ALPs to photons at the level of $10^{-3}{\text{GeV}}^{-1}$. The limits are the most restrictive to date for $0.2\,<\,m_a\,<\,1\,{\text{GeV/$c$}^2}$.

9 pages, 5 figures + supplemental material. Published in Physical Review Letters