A Measurement of the Mass and Full-Width of the $\eta_c$ Meson

TL;DR

This paper measures the mass and width of the $$ meson using $J/$ decays in a large data sample, providing precise values that improve understanding of charmonium states.

Contribution

First precise measurement of the $$ meson mass and width using multiple decay channels in $J/$ decays with a large data set.

Findings

Mass of $$ meson: 2976.6 MeV with uncertainties

Width of $$ meson: 11.0 MeV with uncertainties

Consistent results with previous measurements

Abstract

In a sample of 7.8 million $J/\psi$ decays collected in the Beijing Spectrometer, the process J/$\psi\to\gamma\eta_c$ is observed for five different $\eta_c$ decay channels: $K^+K^-\pi^+\pi^-$, $\pi^+\pi^-\pi^+\pi^-$, $K^\pm K^0_S \pi^\mp$ (with $K^0_S\to\pi^+\pi^-$), $\phi\phi$ (with $\phi\to K^+K^-$) and $K^+K^-\pi^0$. From these signals, we determine the mass of $\eta_c$ to be $2976.6\pm2.9\pm1.3$ MeV. Combining this result with a previously reported result from a similar study using $\psi(2S)\to\gamma\eta_c$ detected in the same spectrometer gives $m_{\eta_c} = 2976.3\pm2.3\pm1.2$ MeV. For the combined samples, we obtain $\Gamma_{\eta_c} = 11.0\pm 8.1\pm 4.1$ MeV.