Tau decay and the structure of the a1

TL;DR

This paper investigates the decay of tau leptons into three pions and a neutrino, modeling the a1 resonance as a dynamically generated meson-molecule, and finds that this approach describes the spectral function effectively with minimal parameters.

Contribution

It introduces a novel approach to modeling the a1 resonance as a coupled-channel meson-molecule, improving the understanding of tau decay spectra.

Findings

Spectral function matches experimental data with one free parameter.

Including an elementary a1 resonance worsens the spectral description.

Dynamical generation of a1 provides a better theoretical framework.

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.