Search for vector charmonium(-like) states in the $e^+e^- \rightarrow \eta J/\psi$ line shape

TL;DR

This study measures the cross section of $e^+e^- ightarrow ext{eta} J/ ext{psi}$, identifies three resonant structures, and attributes them to known charmonium states, providing their parameters and branching fractions.

Contribution

It presents the first detailed measurement and line shape analysis of $e^+e^- ightarrow ext{eta} J/ ext{psi}$, identifying three charmonium(-like) states and their properties.

Findings

Three resonant structures observed in the line shape.

Parameters for $ ext{psi}(4040)$, $Y(4220)$, and $ ext{psi}(4415)$ determined.

Branching fractions for $ ext{psi}(4040)$ and $ ext{psi}(4415)$ decays provided.

Abstract

The cross section of $e^+e^- \rightarrow \eta J/\psi$ has been measured by BESIII and Belle experiments. Fit to the $e^+e^- \rightarrow \eta J/\psi$ line shape, three resonant structures are evident. The parameters for the three resonant structures are $M_{1}=(3980\pm17\pm7)$ MeV/$c^{2}$, $\Gamma_{1}=(104\pm32\pm13)$ MeV; $M_{2}=(4219\pm5\pm4)$ MeV/$c^{2}$, $\Gamma_{2}=(63\pm9\pm3)$ MeV; $M_{3}=(4401\pm12\pm4)$ MeV/$c^{2}$, $\Gamma_{3}=(49\pm19\pm4)$ MeV, where the first uncertainties are statistical and the second systematic. We attribute the three structures to $\psi(4040)$, $Y(4220)$ and $\psi(4415)$ states. The branching fractions $\mathcal{B}(\psi(4040) \rightarrow \eta J/\psi)$ and $\mathcal{B}(\psi(4415) \rightarrow \eta J/\psi)$ are given. If $Y(4220)$ is taken as $\psi(4S)$ state, the branching fraction $\mathcal{B}(\psi(4S) \rightarrow \eta J/\psi)$ is also given. Combining all $Y(4220)$ parameters obtained from different decays, we give average parameters for $Y(4220)$, which are $M_{Y(4220)}=(4220.8\pm2.4)$ MeV/$c^{2}$, $\Gamma_{Y(4220)}=(54.8\pm3.3)$ MeV.