Structure of the rho meson and the pion form-factor

From the electromagnetic form factor of the pion and from the decay modes of the τ lepton involving two or three pions one can see that the isovector–vector quark current couples strongly to the ρ meson whereas the isovector– axial-vector current couples strongly to the a1. Therefore such reactions provide excellent access to studies of the properties of (axial-)vector mesons. It has been shown recently [1] that the τ decay data, where the a1 meson shows up as a bump in the three-pion spectral information, can be well understood by the assumption that initially a ρπ pair is produced which dynamically generates a resonant state in its rescattering process. In the following it is demonstrated with the same method that the ρ cannot be understood as a dynamically generated state of π-π scattering. In other words, while the a1 meson seems to have a hadronic substructure, the ρ meson appears to be dominantly a quark-antiquark state. For further details and references see [2].