Recent BES charmonium results

TL;DR

This paper reports new measurements of charmonium states and decay processes using large samples of $J/\psi$, $\psi(2S)$, and $\psi(3770)$ decays, revealing structures, decay modes, and deviations from theoretical predictions.

Contribution

It presents the first observation of $\chi_{cJ}$ decays to $\Lambda\bar{\Lambda}$ and measures branching ratios for various charmonium decays, challenging existing models.

Findings

Observation of a $p\bar{p}$ mass structure near threshold.

First measurement of $\chi_{cJ} \to \Lambda\bar{\Lambda}$ branching ratios.

Deviation from the pQCD '12% rule' in $K_s^0K_L^0$ decays.

Abstract

Using 58 million $J/\psi$ decays, we have investigated the $p \bar{p}$ invariant mass spectrum in the radiative decay $J/\psi \to \gamma p \bar{p}$ and observe a prominent structure with mass near $2m_p$. Fitting with an S-wave Breit-Wigner, we obtain a peak mass of $M= 1859^{+3}_{-10}(\rm stat)^{+5}_{-25}(sys) MeV/c^2$. $J/\psi \to \gamma \eta_c$ decays from the same sample are used to determine the mass, width, and hadronic branching ratios of the $\eta_c$. From a sample of 14 million $\psi(2S)$ events, the first observation of $\chi_{cJ}$ (J=0,1,2) decays to $\Lambda\bar{\Lambda}$ is made, and branching ratios are determined, which are larger than expected from the Color Octet Model. Branching ratios of $K_s^0K_L^0$ in both $\psi(2S)$ and $J/\psi$ decays are measured, and a more than four sigma deviation from the pQCD predicted "12% rule" is observed. In $\psi(3770)$ decays, evidence for the non-$D\bar{D}$ decay to $\pi^+\pi^-J/\psi$ is observed.