Illuminating hadron structure by scattering light on light

Two photon production of hadronic resonances is one of the clearest ways of revealing their composition, as illustrated in Fig. 1. The nature of the isoscalar scalars seen in ππ scattering below 2 GeV, the f0(600) or σ, f0(980), f0(1370), f0(1510), f0(1720), · · ·, remains an enigma [1,2]. While models abound in which some are qq, some qqqq, sometimes one is a KK-molecule, and one a glueball, definitive statements are few and far between. Since photons couple to neutral hadrons through their charged constituents, as in Fig. 1, their two photon width is a measure of the charge of these [3,4,5,6,7,8]. For instance, if the f0(980) is an ss state its radiative width is 0.2 keV [3], while if it is a KK-molecule, this is 0.6 keV [4] depending on the specifics of the model [6]. Can experiment distinguish these possibilities?