We discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of χPT. Using low-energy data on the pion form factor and the γγ→π+π−(π0π0) cross sections as inputs, we find Γ(σ→γγ)≅0.126 keV. The smallness of the result and the relative weight between its components, Γγγ→S1Γγγ→ππ→S1≤1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.
CitationAmetller, L., Talavera, P. Insights into Q2Qbar2 states from an effective perspective. "Physical review D - Particles, fields, gravitation and cosmology", 07 Octubre 2015, vol. 92, p. 074008-1-074008-10.
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