Search for diphoton decays of an axionlike particle in radiative 𝐽/𝜓 decays

We search for the diphoton decay of a light pseudoscalar axionlike particle, 𝑎, in radiative 𝐽/𝜓 decays, using 1010 𝐽/𝜓 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/𝜓→𝛾⁢𝑎)×ℬ⁡(𝑎→𝛾⁢𝛾) and the axionlike particle photon coupling constant 𝑔𝑎⁢𝛾⁢𝛾 in the ranges of (3.7–48.5)×10−8 and (2.2–101.8)×10−4  GeV−1, respectively, for 0.18≤𝑚𝑎≤2.85  GeV/⁢𝑐2. These are the most stringent limits to date in this mass region.