Search for a light CP-odd Higgs boson in the radiative decays of J/psi

We search for a light Higgs boson $A^0$ in the fully reconstructed decay chain of $J/\psi \rightarrow \gamma A^0$, $A^0 \rightarrow \mu^+\mu^-$ using $(225.0\pm2.8)\times10^6$ $J/\psi$ events collected by the BESIII experiment. The $A^0$ is a hypothetical CP-odd light Higgs boson predicted by many extensions of the Standard Model including two spin-0 doublets plus an extra singlet. We find no evidence for $A^0$ production and set $90\%$ confidence-level upper limits on the product branching fraction $\mathcal{B}(J/\psi \rightarrow \gamma A^0) \times \mathcal{B}(A^0 \rightarrow \mu^+\mu^-)$ in the range of $(2.8-495.3)\times 10^{-8}$ for $0.212 \le m_{A^0} \le 3.0$ GeV/c$^2$. The new limits are $5$ times below our previous results, and the nature of the $A^0$ is constrained to be mostly singlet.