Tau decay and the structure of the a1

Understand

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.