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

TL;DR

This paper reports a search for axion-like particles produced in electron-positron collisions at Belle II, setting new upper limits on their coupling to photons within a specific mass range, with no evidence of their existence found.

Contribution

First search for axion-like particles in $e^+e^-$ collisions at Belle II, establishing the most restrictive limits on their photon coupling for certain masses.

Findings

No evidence for axion-like particles was observed.

Set the most restrictive upper limits on ALP-photon coupling for masses 0.2-1 GeV/c².

Excluded certain parameter space for ALPs at 95% confidence level.

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 ${e^+e^-\to\gamma a, a \to\gamma\gamma}$ 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$}^2}$.