Study of the $\pi\pi$ mass spectra in the process $e^+e^- \to \pi^+\pi^-\pi^0$ at $\sqrt[]{s} \simeq 1020$ MeV

TL;DR

This study analyzes the $ ho$ and $ ho^ ext{0}$ meson spectra in $e^+e^- o ext{3 pions}$ at 1020 MeV, confirming the dominance of the $ ho o ext{2 pions}$ transition and setting limits on non-$ ho ext{pi}$ decay modes.

Contribution

It provides detailed measurements of the $ ho$ meson parameters and the $ ho$-$ ho^ ext{0}$ mass difference, and investigates the $ ho$-$ ho^ ext{0}$ interference at this energy.

Findings

The $ ho$ meson mass is $775.0 ext{ MeV}$ with a width of $150.4 ext{ MeV}$.

Upper limit on $ ext{BR}( ext{phi} o 3 ext{ pions})$ is $6 imes 10^{-4}$.

The $ ho$-$ ho^ ext{0}$ interference was observed at two standard deviations.

Abstract

The invariant mass spectra of the $\pi^+\pi^-$ and $\pi^\pm\pi^0$ pairs in the process $e^+e^- \to \pi^+\pi^-\pi^0$ were studied in the SND experiment at the VEPP-2M collider in the energy region $\sqrt[]{s} \simeq 1020$ MeV. These studies were based on about $0.5 \times 10^6$ experimental events. The spectra were analyzed in the framework of the vector meson dominance model. It was found that the experimental data can be described with $e^+e^- \to \rho\pi \to \pi^+\pi^-\pi^0$ transition only. Upper limit on the branching ratio of the $\phi(1020)\to\pi^+\pi^-\pi^0$ decay through intermediate states different from $\rho\pi$ was obtained at the 90 % confidence level: $B(\phi\to\pi^+\pi^-\pi^0)<6 \cdot 10^{-4}$. The $\rho$-meson mass and width which follow from the spectra analysis are $m_\rho=775.0\pm 1.3$ MeV, $\Gamma_\rho=150.4 \pm 3.0$ MeV. Neutral and charged $\rho$-mesons mass difference was found to equal $m_{\rho^\pm}-m_{\rho^0}=-1.3\pm2.3$ MeV. In the $\pi^+\pi^-$ mass spectrum the $\rho-\omega$ interference was seen at two standard deviations level.