Search for diphoton decays of an axionlike particle in radiative $J/\psi$ decays

We search for the diphoton decay of a light pseudoscalar axionlike particle, $a$, in radiative $J/\psi$ decays, using ${10}^{10}J/\psi$ events collected with the BESIII detector. We find no evidence of a signal and set upper limits at the 95% confidence level on the product branching fraction $B(J/\psi \to \gamma a)\times B(a\to \gamma \gamma )$ and the axionlike particle photon coupling constant $g_{a\gamma \gamma }$ in the ranges of $\left(3.7–48.5\right)\times {10}^{-8}$ and $\left(2.2–101.8\right)\times {10}^{-4}{\mathrm{GeV}}^{-1}$, respectively, for $0.18\le m_a\le 2.85\mathrm{GeV}/c^2$. These are the most stringent limits to date in this mass region.