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 $10^{ -3}\,{ \text{ GeV}^{ -1}}$. The limits are the most restrictive to date for $0.2\,<\,m_a\,<\,1\,{ \text{ GeV/$c$}^2}$.
Categories: hep-ex