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

Understand

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 ${e^+e^-\to\gamma a, a \to\gamma\gamma}$ 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 $g_{a\gamma\gamma}$ of ALPs to photons at the level of $10^{-3}\,{\text{GeV}^{-1}}$.