Pion transition form factor in the Regge approach

Understand

We explore the BaBar puzzle within the Regge approach.

• After reviewing the chiral quark models in applications to PDF and PDA of the pion, we argue that variants of these models, fulfilling the chiral anomaly, may in fact violate the second Terazawa-West unitarity bound, which is based on unverified assumptions for the real part of the amplitude.
• Consequently, the transition form factor need not vanish at large values of the photon virtuality.
• Then we show that the experimental data may be properly explained with incomplete vector-meson dominance in a simple model with one state, as well as in more sophisticated radial Regge models including infinitely many states.