Two-photon width and gluonic component of σ/f0(600)

We analyse data on ππ and γγ scattering below 700 MeV within an improved analytic K-matrix model. This model is based on an effective theory with couplings between resonances, hadrons and photons. The two-photon decay of a resonance can proceed through intermediate transition into charged hadrons (here: π+π−) and their subsequent annihilation or through a “direct” transition into photons. Our analysis confirms the rather large total radiative width of the σ resonance which we find as (3.9±0.6) keV but suggests its dominance by the ππ rescattering process. This process is not sensitive to the internal structure of the resonance contrary to the direct component which we find small, (0.13±0.05)keV, and well consistent with the expectations for an unmixed glueball according to the QCD sum rule calculations.