The spectrum of light isovector mesons with $C=+1$ from the COMPASS experiment

TL;DR

This paper presents a detailed partial wave analysis of diffractively produced three-pion states from the COMPASS experiment, extracting resonance parameters for light mesons and analyzing the isobar shapes with high confidence.

Contribution

It provides the most advanced partial wave analysis of multi-body final states with systematic studies, and performs the first isobar-freed analysis in diffraction to determine isobar shapes.

Findings

Resonance parameters for eleven light mesons were determined with high confidence.

Mass and width of most $a_{J}$ and $ ho_{J}$ states were measured below 2.1 GeV/c^2.

First isobar-freed analysis in diffraction was successfully performed.

Abstract

Based on the largest event sample of diffractively produced $\pi^-\pi^-\pi^+$, obtained by a pion beam of $190~\rm{GeV/c}$ momentum, the COMPASS collaboration has performed the most advanced partial wave analysis on multi-body final states, using the isobar model. The large number of waves included in the analysis reduces truncation effects. We have used fourteen out of a total of eighty-eight waves, to extract resonance parameters for eleven light meson candidates, most of them observed previously. The coherence of the analysis and the large variety of systematic studies has allowed to determine mass and width of most $a_{J}$ and $\pi_{J}$ states with six different values of $J^{PC}$ below a mass of $2.1~\rm{GeV/c^2}$, with high confidence. We exploited that the production rates of resonant and non-resonant contributions in these fourteen waves vary differently with the four-momentum transfer squared in the reaction. In addition, we have performed the first isobar-freed analysis in diffraction, from which we have determined the shape of the $[\pi\pi]_{S-wave}$ isobar for different $J^{PC}$ of the $3\pi$ system.