engleski

Application of Jain and Munczek's bound-state approach to gamma gamma-processes of pi0, eta_c and eta_b

We point out the problems affecting most quark-antiquark bound-state approaches when they are faced with the electromagnetic processes dominated by Abelian axial anomaly. However, these problems are resolved in the consistently coupled Schwinger-Dyson and Bethe-Salpeter approach. Using one of the most successful variants of this approach, we find the dynamically dressed propagators of the light u and d quarks, as well as the heavy c and b quarks, and nd the Bethe-Salpeter amplitudes for their bound states pi^0, eta_c and eta_b. Thanks to incorporating the dynamical chiral symmetry breaking, the pion simultaneously appears as the (pseudo)Goldstone boson. We give the theoretical predictions for the gamma gamma decay widths of pi^0, eta_c and eta_b, and for the pi^0 gamma* -> gamma transition form factor, and compare them with experiment. In the chiral limit, the axial-anomaly result for pi0 -> gamma gamma is reproduced analytically in the consistently coupled Schwinger-Dyson and Bethe-Salpeter approach, provided that the quark-photon vertex is dressed consistently with the quark propagator, so that the vector Ward-Takahashi identity of QED is obeyed. On the other hand, the present approach is also capable of quantitatively describing systems of heavy quarks, concretely eta_c and possibly eta_b, and their gamma gamma-decays. We discuss the reasons for the broad phenomenological success of the bound-state approach of Jain and Munczek.

Nonperturbative QCD; Hadronic structure; Axial anomaly; Electromagnetic processes; Constituent quarks; Current quarks

nije evidentirano