A theory of the J/gY→η(η′)γ decays

It is argued that the $\text{J}\text{gY}\text{→η(η′)γ}$ decay rates are directly calculable within quantum chromodynamics. The calculation of $\text{Γ(}\text{J}\text{Ψ}\text{→ηγ)}$ is most straightforward and involves no new hypotheses as compared to, say, the $\text{J}\text{Ψ}\text{→}\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}$ decay in detail earlier. To the contrary, evaluation of the $\text{J}\text{Ψ}\text{→η′γ}$ width puts on trial dynamical results concerning the η′ coupling to gluons. The latter depends crucially on non-perturbative power terms in the QCD sum rules. The calculated numbers for $\text{Γ(}\text{J}\text{Ψ}\text{→ηγ)}$ and $\text{Γ(}\text{J}\text{Ψ}\text{→η′γ)}$ agree well with the data.