A partial wave analysis of the $\pi^- \pi^- \pi^+$ and $\pi^- \pi^0 \pi^0$ systems and the search for a $J^{PC}=1^{-+}$ meson

TL;DR

This study performs a detailed partial wave analysis of three-pion systems produced in pion-proton interactions at 18 GeV/c, confirming known resonances and finding no evidence for the hypothesized exotic $J^{PC}=1^{-+}$ meson.

Contribution

It provides a comprehensive partial wave analysis with high-statistics data, offering detailed resonance identification and refuting earlier evidence for an exotic meson.

Findings

Identified known resonances such as $a_1(1260)$ and $a_2(1320)$

No evidence found for the $J^{PC}=1^{-+}$ exotic meson

Confirmed stability of partial wave fits

Abstract

A partial wave analysis (PWA) of the $\pi^- \pi^- \pi^+$ and $\pi^- \pi^0 \pi^0$ systems produced in the reaction $\pi^- p \to (3\pi)^-p$ at 18 GeV/$c$ was carried out using an \emph{isobar} model assumption. This analysis is based on 3.0M $\pi^- \pi^0 \pi^0$ events and 2.6M $\pi^- \pi^-\pi^+$ events and shows production of the $a_1(1260)$, $a_2(1320)$, $\pi_2(1670)$ and $a_4(2040)$ resonances. Results of detailed studies of the stability of partial wave fits are presented. An earlier analysis of 250K $\pi^- \pi^- \pi^+$ events from the same experiment showed possible evidence for a $J^{PC}=1^{-+}$ exotic meson with a mass of $\sim$1.6 GeV/$c^2$ decaying into $\rho \pi$. In this analysis of a higher statistics sample of the $(3\pi)^-$ system in two charged modes we find no evidence of an exotic meson.