Tau decay and the structure of the a1

Abstract

We analyse the decay $\tau \to \pi \pi \pi \nu$ based on the recently developed techniques to generate axial-vector resonances dynamically. Under the assumption that the a1 is a coupled-channel meson-molecule, the spectral function is described surprisingly well by adjusting only one free parameter. Including, in addition, an elementary a1 corrupts the results.

Comments: Replaced figures, due to an error in the calculation, which we corrected. (Conclusions did not change.) Dropped last part discussing higher order effect in favour of further elucidation