PubDB | BELLE2-PUB-PH-2025-062

Abstract	Axion-like particles (ALPs) are predicted in many extensions of the Standard Model and provide a well-motivated portal between visible and hidden sectors through their coupling to photons. We search for ALPs produced in the process $e^{+}e^{-}\to\gamma a$, $a\to\gamma\gamma$, using a data sample corresponding to an integrated luminosity of \lumiinvfb{} recorded by the \belletwo detector at the SuperKEKB $e^{+}e^{-}$ collider. Events containing three photons are used to reconstruct the ALP as a narrow peak in the di-photon invariant mass spectrum over the range $0.17 < m_{a} < 9.80~\mathrm{GeV}/c^{2}$. No significant excess above the background expectation is observed, with the largest local (global) significance of $3.3\sigma$ ($1.4\sigma$). We set $95\%$ confidence level upper limits on the production cross section and on the ALP-photon coupling $g_{a\gamma\gamma}$, reaching sensitivities at the level of $10^{-4}~\mathrm{GeV}^{-1}$. The limits are for the most ALP mases the most restrictive to date in the mass range $0.17 < m_{a} < 5.00~\mathrm{GeV}/c^{2}$ and improve over previous searches by up to one order of magnitude.
Tags	Run 1 (partial)
Working group	Dark Sector and Low Multiplicity
Principal authors	@ Giacomo De Pietro, Torben Ferber, Alexander Heidelbach
Reference	submitted to PRL
Links	arXiv:2607.07800, Inspire,

Abstract

Axion-like particles (ALPs) are predicted in many extensions of the Standard Model and provide a well-motivated portal between visible and hidden sectors through their coupling to photons. We search for ALPs produced in the process $e^{+}e^{-}\to\gamma a$, $a\to\gamma\gamma$, using a data sample corresponding to an integrated luminosity of \lumiinvfb{} recorded by the \belletwo detector at the SuperKEKB $e^{+}e^{-}$ collider. Events containing three photons are used to reconstruct the ALP as a narrow peak in the di-photon invariant mass spectrum over the range $0.17 < m_{a} < 9.80~\mathrm{GeV}/c^{2}$. No significant excess above the background expectation is observed, with the largest local (global) significance of $3.3\sigma$ ($1.4\sigma$). We set $95\%$ confidence level upper limits on the production cross section and on the ALP-photon coupling $g_{a\gamma\gamma}$, reaching sensitivities at the level of $10^{-4}~\mathrm{GeV}^{-1}$. The limits are for the most ALP mases the most restrictive to date in the mass range $0.17 < m_{a} < 5.00~\mathrm{GeV}/c^{2}$ and improve over previous searches by up to one order of magnitude.

Search for ALPs in $e^+e^-\to\gamma a,\, a\to\gamma\gamma$ at Belle II