Search for Axion-Like Particles Decaying to Two Photons at Belle II

Sumitted to PubDB: 2026-11-02

Category: Phd Thesis

Principal Authors	Alexander Heidelbach
Date	2025-11-28
Belle II Number	BELLE2-PTHESIS-2026-029
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. I 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 $408~\mathrm{fb}^{-1}$ recorded by the Belle~II 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 background is observed. I 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 the most restrictive to date over nearly the entire mass range $0.17 < m_a < 5.00~\mathrm{GeV}/c^2$, and improve upon previous results by up to a factor 9.
Institute	Karlsruhe