Measurements of the Mass and Full-Width of the $\eta_c$ Meson

TL;DR

This paper reports precise measurements of the mass and width of the $ ext{eta}_c$ meson using a large dataset of $J/ ext{psi}$ decays observed across five different channels, providing key parameters for understanding charmonium states.

Contribution

The study presents the first combined measurement of the $ ext{eta}_c$ meson's mass and width from multiple decay channels with high statistical significance.

Findings

Mass of $ ext{eta}_c$ = 2977.5 ± 1.0 (stat.) ± 1.2 (syst.) MeV/$c^2$

Width of $ ext{eta}_c$ = 17.0 ± 3.7 (stat.) ± 7.4 (syst.) MeV/$c^2$

Observation of $ ext{eta}_c$ in five decay channels

Abstract

In a sample of 58 million $J/\psi$ events collected with the BES II detector, the process J/$\psi\to\gamma\eta_c$ is observed in five different decay channels: $\gamma K^+K^-\pi^+\pi^-$, $\gamma\pi^+\pi^-\pi^+\pi^-$, $\gamma K^\pm K^0_S \pi^\mp$ (with $K^0_S\to\pi^+\pi^-$), $\gamma \phi\phi$ (with $\phi\to K^+K^-$) and $\gamma p\bar{p}$. From a combined fit of all five channels, we determine the mass and full-width of $\eta_c$ to be $m_{\eta_c}=2977.5\pm1.0 ({stat.})\pm1.2 ({syst.})$ MeV/$c^2$ and $\Gamma_{\eta_c} = 17.0\pm3.7 ({stat.})\pm7.4 ({syst.})$ MeV/$c^2$.