Assessment of the a2(1320) tensor-meson contribution to η/η′→π0γγ decays

In light of the recent measurement of the η→π0γγ decay by the KLOE-2 Collaboration, a previous analysis including vector- and scalar-meson exchange contributions using the vector meson dominance and linear sigma model frameworks, respectively, is extended in the present study to incorporate the effects of the a2(1320) tensor meson within a chiral context. Although the individual contribution of the a2 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 mγγ2 values. The total decay rate is calculated to be Γ(η→π0γγ)=0.154(22) eV, which corresponds to a branching ratio of BR(η→π0γγ)=1.17(17)×10-4. This result is approximately 5σ below the reported value of the particle data group, 2.55(22)×10-4, while it is in very good agreement with the KLOE-2 measurement, 0.98(11stat)(14syst)×10-4. In contrast, the total contribution of the a2 is found to be negligible in η′→π0γγ, as this process is completely dominated by the exchange of an on-shell ω vector resonance.