Can the gamma-gamma processes reveal the nature of the sigma ?

Abstract

We reanalyse the gamma-gamma scattering data and conclude that in the mass region below 1 GeV the cross section for gamma-gamma to pi^0-pi^0 can be largely explained by the one pion exchange process with pi-pi rescattering. The radiative width of the sigma is estimated and a model dependent separation into contributions from direct gamma-gamma decay and decay through rescattering is obtained. We confront these findings with QCD spectral sum rule (QSSR) predictions and conclude that the sigma can have a large gluonium component in its wave function.