Observation of the $X(2370)$ and search for the $X(2120)$ in $J/\psi\to\gamma K\bar{K} \eta'$

TL;DR

This study reports the observation of the $X(2370)$ resonance in $J/ar{J}$ decays with precise measurements of its properties, while setting upper limits on the $X(2120)$, advancing understanding of these states in particle physics.

Contribution

First observation and detailed measurement of the $X(2370)$ resonance parameters in $J/ar{J}$ decays, and the first search setting upper limits for the $X(2120)$ in the same decay channel.

Findings

$X(2370)$ observed with 8.3$\sigma$ significance.

Resonance parameters of $X(2370)$ measured as $M=2341.6 ext{MeV}/c^2$, $\Gamma=117 ext{MeV}$.

No significant signal for $X(2120)$; upper limits set on its production.

Abstract

Using a sample of $1.31\times10^{9} ~J/\psi$ events collected with the BESIII detector, we perform a study of $J/\psi\to\gamma K\bar{K}\eta'$. The $X(2370)$ is observed in the $K\bar{K}\eta'$ invariant-mass distribution with a statistical significance of 8.3$\sigma$. Its resonance parameters are measured to be $M = 2341.6 \pm 6.5\text{(stat.)} \pm 5.7\text{(syst.)}$~MeV/$c^{2}$ and $\Gamma = 117 \pm 10 \text{(stat.)} \pm 8 \text{(syst.)}$~MeV. The product branching fractions for $J/\psi\to \gamma X(2370), X(2370) \to K^{+} K^{-}\eta'$ and $J/\psi\to \gamma X(2370), X(2370) \to K_{S}^{0} K_{S}^{0}\eta'$ are determined to be $(1.79 \pm 0.23 \text{(stat.)} \pm 0.65 \text{(syst.)}) \times 10^{-5}$ and $(1.18 \pm 0.32\ text{(stat.)} \pm 0.39 \text{(syst.)}) \times 10^{-5}$, respectively. No evident signal for the $X(2120)$ is observed in the $K\bar{K}\eta'$ invariant-mass distribution. The upper limits for the product branching fractions of $\mathcal{B}(J/\psi \to \gamma X(2120)\to\gamma K^{+} K^{-} \eta')$ and $\mathcal{B}(J/\psi\to\gamma X(2120)\to\gamma K_{S}^{0} K_{S}^{0} \eta')$ are determined to be $1.49\times10^{-5}$ and $6.38\times10^{-6}$ at the 90% confidence level, respectively.