Perturbative QCD versus pion exchange and hadronic FSI effects in the gamma gamma --> pi^+ pi^- reaction

Abstract

The interplay of pQCD, pion exchange and FSI effects is studied for the gamma gamma --> pi^+ pi^- reaction in the region of 2 GeV < W_{gamma gamma} < 6 GeV. We find strong interference effects between pQCD and soft pion-exchange amplitudes up to W_{gamma gamma} ~ 4 GeV. We discuss to which extend the conventional hadronic FSI effects could cloud the pQCD effects. We study multipole soft and hard scattering effects as well as the coupling between final state hadronic channels. We show how the perturbative effects in gamma gamma --> rho rho may mix with perturbative effects in gamma gamma --> pi^+ pi^-. The effects discussed in this paper improve the agreement with the new data of the DELPHI and ALEPH collaborations. We give estimates of the onset of the pQCD regime. Predictions for gamma gamma --> pi^0 pi^0 are presented.