Observation of $e^{+}e^{-} \to \eta J/\psi$ at center-of-mass energy $sqrt{s}=4.009$ GeV

TL;DR

This paper reports the first observation of the process $e^{+}e^{-} o exteta J/ extpsi$ at 4.009 GeV, measuring its cross section and exploring related transitions with high statistical significance.

Contribution

It provides the first measurement of $e^{+}e^{-} o exteta J/ extpsi$ production at this energy and estimates the transition rate from $ ext{psi}(4040)$ to $ exteta J/ extpsi$.

Findings

First observation of $e^{+}e^{-} o exteta J/ extpsi$ with >10σ significance.

Measured cross section of $(32.1 extpm 2.8 extpm 1.3)$ pb.

Upper limit set for $e^{+}e^{-} o extpi^0 J/ extpsi$ process.

Abstract

Using a 478 pb$^{-1}$ data sample collected with the BESIII detector operating at the BEPCII storage ring at a center-of-mass energy of $\sqrt{s}=4.009$ GeV, the production of $e^{+}e^{-}\to \eta J\psi$ is observed for the first time with a statistical significance of greater than $10\sigma$. The Born cross section is measured to be $(32.1\pm 2.8 \pm 1.3)$ pb, where the first error is statistical and the second systematic. Assuming the $\eta J\psi$ signal is from a hadronic transition of the $\psi(4040)$, the fractional transition rate is determined to be ${\cal B}(\psi(4040)\to \eta J\psi)=(5.2\pm 0.5\pm 0.2\pm 0.5)\times 10^{-3}$, where the first, second, and third errors are statistical, systematic, and the uncertainty from the $\psi(4040)$ resonant parameters, respectively. The production of $e^{+}e^{-} \to \pi^0 J\psi$ is searched for, but no significant signal is observed, and ${\cal B}(\psi(4040)\to \pi^0 J\psi) < 2.8 \times 10^{-4}$ is obtained at the 90% confidence level.