Search for axionlike particles produced in e⁺e⁻ collisions at Belle II

Abstract

We present a search for the direct production of a light pseudoscalar α decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e⁺e⁻→γα, α→γγ in the mass range 0.2<mₐ<9.7 GeV/c² using data corresponding to an integrated luminosity of (445±3) pb⁻¹. Light pseudoscalars interacting predominantly with standard model gauge bosons (so-called axionlike 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 gₐγγ of ALPs to photons at the level of 10⁻³ GeV⁻¹. The limits are the most restrictive to date for 0.2<mₐ<1 GeV/c².