Improved measurement of the branching fraction of h c → γη′/η and search for h c → γπ 0

The processes h c → γP (P = η′, η, π 0) are studied with a sample of (27.12 ± 0.14) × 108 ψ(3686) events collected by the BESIII detector at the BEPCII collider. The decay h c → γη is observed for the first time with the significance of 9.0 σ, and the branching fraction is determined to be (3.77 ± 0.55 ± 0.13 ± 0.26) × 10 −4, while $B$ (h c → γη′) is measured to be (1.40 ± 0.11 ± 0.04 ± 0.10) × 10 −3, where the first uncertainties are statistical, the second systematic, and the third from the branching fraction of ψ(3686) → π 0 h c . The combination of these results allows for a precise determination of ${R}_{{h}_{c}}=\frac{B\left({h}_{c}\to \mathrm{\gamma \eta }\right)}{B\left({h}_{c}\to \gamma {\eta }^{\prime }\right)},$ which is calculated to be (27.0 ± 4.4 ± 1.0)%. The results are valuable for gaining a deeper understanding of η − η′ mixing, and its manifestation within quantum chromodynamics. No significant signal is found for the decay h c → γπ 0, and an upper limit is placed on its branching fraction of $B$ (h c → γπ 0) < 5.0 × 10 −5, at the 90% confidence level.