We search for a light Higgs boson A0 in the fully reconstructed decay chain of J/ψ→γA0, A0→μ+μ- using (225.0±2.8)×106 J/ψ events collected by the BESIII experiment. The A0 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 A0 production and set 90% confidence-level upper limits on the product branching fraction B(J/ψ→γA0)×B(A0→μ+μ-) in the range of (2.8-495.3)×10-8 for 0.212≤mA0≤3.0 GeV/c2. The new limits are five times below our previous results, and the nature of the A0 is constrained to be mostly singlet.
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