Pion transition form factor in the Regge approach

TL;DR

This paper investigates the pion transition form factor using the Regge approach, challenging existing bounds and demonstrating that experimental data can be explained with models involving incomplete vector-meson dominance and multiple states.

Contribution

It shows that variants of chiral quark models can violate the Terazawa-West bound, allowing the form factor to remain non-zero at high photon virtuality, and applies Regge models to fit experimental data.

Findings

Models with incomplete vector-meson dominance fit the data well.

The Terazawa-West bound may not hold under certain model assumptions.

Experimental constraints from Z -> pi0 gamma decay are satisfied.

Abstract

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. We also consider the experimental constraint from the rare Z -> pi0 gamma decay, which is comfortably satisfied in our approach. Finally, we point out that the photon momentum asymmetry parameter may noticeably influence the precision fits to the data.