Pion transition form factor in the Regge approach and incomplete vector-meson dominance

摘要

The concept of incomplete vector-meson dominance and Regge models is appliedto the transition form factor of the pion. First, we argue that variants of thechiral quark model fulfilling the chiral anomaly may violate the Terazawa-Westunitarity bounds, as these bounds are based on unverified assumptions for thereal parts of the amplitudes, precluding a possible presence of polynomialterms. A direct consequence is that the transition form factor need notnecessarily vanish at large values of the photon virtuality. Moreover, in therange of the BaBar experiment, the Terazawa-West bound is an order of magnitudeabove the data, thus is of formal rather than practical interest. Then wedemonstrate how the experimental data may be properly explained with incompletevector-meson dominance in a simple model with one state, as well as in moresophisticated Regge models. Generalizations of the simple Regge model along thelines of Dominguez result in a proper description of the data, where one mayadjust the parameters in such a way that the Terazawa-West bound is satisfiedor violated. We also impose the experimental constraint from the Z -> pi0 gammadecay. Finally, we point out that the photon momentum asymmetry parameter maynoticeably influence the precision analysis.