Vector meson dominance and the pi^0 transition form factor

TL;DR

This paper demonstrates that the pi^0 transition form factor significantly deviates from its one-photon limit even at small Q^2, highlighting the importance of considering both photon virtualities in experimental and theoretical analyses.

Contribution

The authors develop a vector-meson-dominance based phenomenological model for the pi^0 transition form factor, accurately describing decay rates and data across various photon virtualities.

Findings

Form factor differs from the one-photon limit at small Q^2.

Model accurately describes pi^0 decay rates without free parameters.

Comparison with data shows good agreement across Q^2 values.

Abstract

It is shown that the pi^0 transition form factor F(Q_1^2,Q_2^2) differs substantially from its one-real-photon limit F(Q_1^2,0) even for rather small values of Q_2^2 (approx 0.1 GeV^2), which cannot be excluded in experiments with one "untagged" electron. It indicates that the comparison of data with theoretical calculations, which usually assume Q_2^2=0, may be untrustworthy. Our phenomenological model of the pi^0 transition form factor is based on the vector-meson-dominance hypothesis and all its parameters are fixed by using the experimental data on the decays of vector mesons. The model soundness is checked in the two-real-photon limit, where it provides a good parameter-free description of the pi^0 -> 2 gamma decay rate, and in the pi^0 Dalitz decay. The dependence of F(Q_1^2,Q_2^2) on Q_1^2 at several fixed values of Q_2^2 is presented and the comparison with existing data performed.