Testing the consistency of the $\omega\pi$ transition form factor with unitarity and analyticity

Abstract

We perform a dispersive analysis of the $\omega\pi$ electromagnetic transition form factor, using as input the discontinuity provided by unitarity below the $\omega\pi$ threshold and including for the first time experimental data on the modulus measured from $e^+e^-\to\omega\pi^0$ at higher energies. The input leads to stringent parameterization-free constraints on the modulus of the form factor below the $\omega\pi$ threshold, which are in disagreement with some experimental values measured from $\omega\to \pi^0\gamma^*$ decay. We discuss the dependence on the input parameters in the unitarity relation, using for illustration an $N/D$ formalism for the P partial wave of the scattering process $\omega\pi \to \pi\pi$, improved by a simple prescription which simulates the rescattering in the crossed channels. Our results confirm the existence of a conflict between experimental data and theoretical calculations of the $\omega\pi$ form factor in the region around 0.6 GeV and bring further arguments in support of renewed experimental efforts to measure more precisely the $\omega\to\pi^0\gamma^*$ decay.