Insights into $Q^2 \bar{Q}^2$ states from an effective perspective

We discuss the two photon coupling of the lightest scalar meson on the basis of an extension of $χ$PT. Using low energy data on the pion form-factor and the $γγ\to π^+π^-(π^0π^0)$ cross-sections as inputs, we find $Γ(σ\toγγ) \cong 0.126~\rm{keV}$. The smallness of the result and the relative weight between its components, ${Γ_{γγ\to S_1}\over Γ{ γγ\toππ\to S_1}} < 1$, suggests that the scalar $0^{++}$ meson is mainly a $Q^2\bar{Q}^2$ state.

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