Multibody decay analyses -- a new phenomenological model for meson-meson subamplitudes

TL;DR

This paper introduces a new phenomenological model for meson-meson interactions up to 2 GeV, improving upon the outdated isobar model by incorporating modern QCD insights, chiral symmetry, and coupled channels.

Contribution

The authors develop a comprehensive, QCD-compatible meson-meson amplitude model that surpasses the traditional isobar model, especially for scalar resonances and coupled channels.

Findings

The new model aligns well with experimental data for the $\

Provides a clear explanation for the sharp phase rise around 1 GeV in $\

Demonstrates compatibility with chiral symmetry and unitarity principles.

Abstract

Meson-meson amplitudes are important on their own and also play key roles in analyses of heavy-meson and tau decays. In this work we propose a new phenomenological model suited to all $SU(3)$ mesonic two-body final state interactions up to energies around 2 GeV. It is aimed at replacing those entering the old isobar model, produced in the 1960's, long before the development of QCD. The only similarity between our new proposal and amplitudes used in the isobar model concern vector resonances in the elastic regime. In other situations, especially those involving scalar resonances and coupled channels, the isobar model is not compatible with post-QCD dynamics. In order to support these claims convincingly and to motivate our approach, we consider applications to the $\pi \pi $ amplitude and compare our version with the isobar model in several different instances. We also show that the new model provides a clear indication of the mechanism responsible for the sharp rise observed in the $\pi\pi$ phase around $1\,$ GeV. The phenomenological amplitudes proposed here are suited to any number of resonances in a given channel and rely just on masses and coupling constants as free parameters. Concerning theory, they incorporate chiral symmetry at low energies, include coupled channels and respect unitarity whenever appropriate.