Observation of $\psi(3770)\to\eta J/\psi$

TL;DR

This paper reports the first observation of the decay $oxed{ ext{psi(3770)} o ext{eta J/psi}}$ at 3.773 GeV, measuring its cross section and branching fraction, providing new insights into charmonium decay processes.

Contribution

The study provides the first experimental evidence of $oxed{ ext{psi(3770)} o ext{eta J/psi}}$ decay and measures its cross section and branching fraction with statistical significance.

Findings

First observation of $oxed{ ext{psi(3770)} o ext{eta J/psi}}$ decay.

Measured cross section at 3.773 GeV: ($8.89 o0.88 o0.42$) pb.

Branching fraction estimated as $(11.6 o6.1 o1.0) imes 10^{-4}$ (coherent fit).

Abstract

The Born cross section of the process $e^+e^-\to\eta J/\psi$ at a center-of-mass energy $\sqrt{s}=3.773 ~\mathrm{GeV}$ is measured to be ($8.89\pm0.88\pm0.42$)$~\mathrm{pb}$, using a data sample collected with the BESIII detector operating at the BEPCII storage ring. The decay $\psi(3770)\rightarrow\eta J/\psi$ is observed for the first time with a statistical significance of $7.4\sigma$. From a fit to the dressed cross-section line-shape of $e^+e^-\to\eta J/\psi$ from $\sqrt{s}=3.773$ to $4.600~\mathrm{GeV}$ we obtain the branching fraction of the decay $\psi(3770)\to \eta J/\psi$ to be $(11.6\pm6.1\pm1.0)\times 10^{-4}$ when the $\psi(3770)$ decay amplitude is added coherently to the other contributions, and $(7.9\pm1.0\pm0.7)\times 10^{-4}$ when it is added incoherently. Here the first uncertainties are statistical and the second are systematic.