Assessment of the $a_{2}(1320)$ tensor-meson contribution to $\eta/\eta^{\prime}\to\pi^{0}\gamma\gamma$ decays

Abstract

In light of the recent measurement of the $\eta\to\pi^{0}\gamma\gamma$ decay by the KLOE-2 Collaboration, a previous analysis including vector- and scalar-meson exchange contributions using the VMD and L$\sigma$M frameworks, respectively, is extended in the present study to incorporate the effects of the $a_{2}(1320)$ tensor meson within a chiral context. Although the individual contribution of the $a_{2}$ is negligible, its destructive interference with the vector-meson resonances is found to be significant, representing approximately $18\%$ of the total signal and substantially affecting the diphoton invariant-mass distribution, especially at low $m_{\gamma\gamma}^{2}$ values. The total decay rate is calculated to be $\Gamma(\eta\to\pi^{0}\gamma\gamma)=0.154(22)$ eV, which corresponds to a branching ratio of ${\rm BR}(\eta\to\pi^{0}\gamma\gamma)=1.17(17)\times10^{-4}$. This result is approximately $5\sigma$ below the reported value of the PDG, $2.55(22)\times10^{-4}$, while it is in very good agreement with the KLOE-2 measurement, $0.98(11_{\rm stat})(14_{\rm syst})\times10^{-4}$. In contrast, the total contribution of the $a_{2}$ is found to be negligible in $\eta^{\prime}\to\pi^{0}\gamma\gamma$, as this process is completely dominated by the exchange of an on-shell $\omega$ vector resonance.