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X. Dong

Publications and source records attributed to X. Dong.

At least 19 recordsLinked to original sources
arXiv2026

Study of $K_{S}^{0}$-$K_{L}^{0}$ asymmetry in the decays $D^0 \to K_{S}^{0}\omega $ and $D^0 \to K_{L}^{0} \omega $

Based on $e^+ e^-$ annihilation data corresponding to an integrated luminosity of 7.93~$fb^{-1}$ collected at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider, the absolute branching fractions of the decays $D^0 \to K_{S}^{0} \omega$ and $D^0 \to K_{L}^{0} \omega $ are measured to be $(11.79 \pm 0.19 \pm 0.26 \pm 0.47) \times 10^{-3}$ and ($10.84 \pm 0.14 \pm 0.23 \pm 0.44) \times 10^{-3}$, respectively. The $K_{S}^{0}- K_{L}^{0}$ branching-fraction asymmetry of these two decays is $R(D^0,K_{S,L}^{0} \omega) = \frac{\mathcal{B}(D^0 \to K_{S}^{0} \omega) - \mathcal{B}(D^0 \to K_{L}^{0}\omega )}{\mathcal{B}(D^0 \to K_{S}^{0} \omega) + \mathcal{B}(D^0 \to K_{L}^{0} \omega)} =(4.2 \pm 1.0 \pm 0.9 \pm 2.8)\%$. Here, the first uncertainties are statistical, the second systematic, and the third arise from the interference between $D^0 \to K_{S,L}^{0} \omega$ and the non-resonant $D^0 \to \pi^+ \pi^- \pi^0 K_{S,L}^{0}$ processes.

hep-ex↗
arXiv2026

Search for the baryonic decay $ D_{s}^{*+} \to \ p \bar{n} $

The first search for the baryonic decay $ D_{s}^{*+} \to \ p \bar{n} $ is performed using $e^+e^-$ collision data taken at center-of-mass energies between 4.128 and 4.226 GeV, collected by the BESIII experiment and corresponding to an integrated luminosity of 7.33 fb$^{-1}$. No significant signal is observed, and an upper limit on the branching fraction is set to be $1.3\times 10^{-4}$ at the $90\%$ confidence level.

hep-ex↗
arXiv2026

Observation of $\psi(3686)\to p K^- K_S^0 \bar \Xi^0+c.c.$

Using a sample of $(2.712 \pm 0.014) \times 10^{9}$ $\psi(3686)$ events collected with the BESIII detector, the decay of $\psi(3686)\to p K^- K_S^0 \bar \Xi^0+c.c.$ is observed for the first time with a statistical significance of $11.5\sigma$. The branching fraction of this decay is measured to be $(2.84\pm 0.40\pm 0.25) \times 10^{-6}$, where the first and second uncertainties are statistical and systematic, respectively. This measurement extends the experimental information on rare multi-strange $\psi(3686)$ decays and provides an experimental reference for future studies of related decay modes.

hep-ex↗
arXiv2026

Measurement of inelastic scattering $\Lambda(\overline{\Lambda})+p\to\Sigma^{0}(\overline{\Sigma}^{0})+p$ via $e^+e^-\to J/\psi\to\Lambda\overline{\Lambda}$

Using a sample of $(10087\pm44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector, we investigate the inelastic scattering processes $\Lambda+p\to\Sigma^{0}+p$ and $\overline{\Lambda}+p\to\overline{\Sigma}^{0}+p$, exploiting hyperons from $J/\psi\to\Lambda\overline{\Lambda}$ decays as an effective beam and the beam-pipe materials as targets. The processes $\Lambda+{}^{9}\mathrm{Be}\to\Sigma^{0}+p+{}^{8}\mathrm{Li}$ and $\overline{\Lambda}+{}^{9}\mathrm{Be}\to\overline{\Sigma}^{0}+p+{}^{8}\mathrm{Li}$ are measured at a hyperon momentum of $1.074~\mathrm{GeV}/c$, with cross sections of $(10.1\pm1.4_{\rm stat}\pm0.7_{\rm syst})$ mb and $(1.7\pm0.6_{\rm stat}\pm0.4_{\rm syst})$ mb, respectively. Under the assumption of surface-dominated hyperon-nucleus scattering, these measurements are used to extract the corresponding proton-target cross sections. Independently, direct measurements using the hydrogen component of the beam-pipe oil yield $(3.2\pm1.1_{\rm stat}\pm0.5_{\rm syst})$ mb for $\Lambda+p\to\Sigma^{0}+p$ and $(1.5\pm0.5_{\rm stat}\pm0.1_{\rm syst})$ mb for $\overline{\Lambda}+p\to\overline{\Sigma}^{0}+p$, consistent with the indirect determinations. The combined cross sections are $(4.7\pm0.7)$ mb and $(1.1\pm0.3)$ mb, respectively. The $\overline{\Lambda}+p\to\overline{\Sigma}^{0}+p$ signal constitutes the first evidence for anti-hyperon inelastic scattering with baryonic final states, with a significance of $3.1\sigma$. The pronounced difference between the $\Lambda p$ and $\overline{\Lambda}p$ inelastic scattering cross sections provides new experimental constraints on hyperon-nucleon and anti-hyperon-nucleon interactions.

hep-ex↗
arXiv2026

Evidence for $\eta_{c}(2S)\to p\bar{p}\pi^{+}\pi^{-}\pi^{0}$ and observation of $\chi_{cJ} \to p\bar{p}\pi^{+}\pi^{-}\pi^{0}$

Using $(2.712\pm0.014)\times 10^9$ $\psi(3686)$ events collected by the BESIII detector at the BEPCII collider, the $\psi(3686) \to \gamma p\bar{p}\pi^+\pi^-\pi^0$ process is investigated. Evidence for the decay of $\eta_{c}(2S)\to p\bar{p}\pi^{+}\pi^{-}\pi^{0}$ is found with a signal significance of 3.3$\sigma$. The product of branching fractions of $\mathcal{B}[\psi(3686)\to \gamma \eta_{c}(2S)]\times\mathcal{B}[\eta_{c}(2S)\to p\bar{p}\pi^{+}\pi^{-}\pi^{0}]$ is determined to be $(3.4\pm0.5\pm0.8) \times 10^{-6}$, where the first uncertainty is statistical and the second systematic. The hadronic decays of $\chi_{cJ} \to p\bar{p}\pi^+\pi^-\pi^0$$~(J=0,1,2)$ are observed, and their branching fractions are measured to be $\mathcal{B}(\chi_{c0}\to p\bar{p}\pi^{+}\pi^{-}\pi^{0})=(4.79\pm 0.01\pm0.40) \times 10^{-3}$, $\mathcal{B}(\chi_{c1}\to p\bar{p}\pi^{+}\pi^{-}\pi^{0})=(2.13\pm 0.01\pm0.17) \times 10^{-3}$, and $\mathcal{B}(\chi_{c2}\to p\bar{p}\pi^{+}\pi^{-}\pi^{0})=(3.72\pm 0.01\pm0.29) \times 10^{-3}$, respectively. Furthermore, the branching fractions for the intermediate processes $\chi_{cJ}\to p\bar{p}\omega$ are updated with significantly improved precision: $\mathcal{B}(\chi_{c0}\to p\bar{p}\omega)=(5.76\pm0.01\pm0.42)\times10^{-4}$, $\mathcal{B}(\chi_{c1}\to p\bar{p}\omega)=(1.85\pm0.01\pm0.13)\times10^{-4}$, and $\mathcal{B}(\chi_{c2}\to p\bar{p}\omega)=(4.51\pm0.01\pm0.33)\times10^{-4}$, respectively.

hep-ex↗
arXiv2026

Measurement of Branching Fraction and Transition Magnetic Moment of the Hyperon Dalitz Decay $\Sigma^0 \rightarrow \Lambda e^+e^-$

Based on a data sample of 10 billion $J/\psi$ events collected with the BESIII detector operating at the BEPCII collider, the Dalitz decay $\Sigma^0 \rightarrow \Lambda e^+e^-$ is studied experimentally for the first time. The $\Sigma^0$ hyperons are produced through the process $J/\psi \rightarrow \Sigma^0\bar{\Sigma}^0$ and analyzed using a double-tag method. The absolute branching fraction is measured to be $\mathcal{B}(\Sigma^0 \rightarrow \Lambda e^+e^-) = (6.34 \pm 0.25_{\rm stat.} \pm 0.23_{\rm syst.}) \times 10^{-3}$. This result shows a $2\sigma$ discrepancy from the theoretical calculation quoted in the PDG, where the uncertainties are statistical and systematic, respectively. In addition to the branching fraction, the transition magnetic moment $\mu$ is determined to be $(1.74 \pm 0.03_{\rm stat.} \pm 0.09_{\rm syst.})\,\mu_N$, where $\mu_N=e/(2m_p)$ represents the nucleon magnetic moment, providing valuable insight into the intrinsic structure of the $\Sigma^0$ hyperon.

hep-ex↗
arXiv2026

First measurements of the branching fractions of $J/\psi$ and $\psi(3686) \to \Sigma^{0} \bar{\Sigma}^{0}\eta$

Based on $(10087 \pm 44) \times 10^6$ $J/\psi$ and $(2712 \pm 14) \times 10^6$ $\psi(3686)$ events collected with the BESIII detector at the BEPCII collider, the hadronic decays $J/\psi \to \Sigma^{0} \bar{\Sigma}^{0} \eta$ and $\psi(3686) \to \Sigma^{0} \bar{\Sigma}^{0} \eta$ are observed for the first time. The corresponding branching fractions are measured to be $\mathcal{B}(J/\psi \to \Sigma^{0} \bar{\Sigma}^{0}\eta)= (7.5 \pm 0.3 \pm 0.8) \times 10^{-5}$ and $\mathcal{B}(\psi(3686) \to \Sigma^{0} \bar{\Sigma}^{0}\eta)= (1.3\pm 0.1 \pm 0.1) \times 10^{-5}$, respectively, where the first uncertainties are statistical, and the second systematic. The ratio $\text{Q} \approx \frac{\mathcal{B}(\psi(3686) \to \Sigma^{0} \bar{\Sigma}^{0} \eta)}{\mathcal{B}(J/\psi \to \Sigma^{0} \bar{\Sigma}^{0} \eta)}$ is determined to be $(17.3 \pm 1.5 \pm 1.7)\%$, which is con sistent with the 12\%-rule within 3.0$\sigma$.~No significant intermediate states or threshold enhancements are observed in the $\Sigma^0$($\bar{\Sigma}^{0}$)$\eta$ and $\Sigma^0$$\bar{\Sigma}^{0}$ invariant mass spectra.

hep-ex↗
arXiv2026

High-precision measurement of the space-like $\eta^\prime$ transition form factor

Using a data sample corresponding to an integrated luminosity of $20.3\ \text{fb}^{-1}$, collected with the BESIII detector at a center-of-mass energy of $3.773\ \text{GeV}$ at the BEPCII collider, we report a precision measurement of the product $Q^2|F(Q^2)|$, where $F(Q^2)$ is the single-virtual space-like transition form factor of the $\eta'$ meson and $Q^2$ is the squared momentum transfer of the tagged virtual photon. The transition form factor is extracted from the differential Born cross section of the two-photon fusion processes $e^+e^- \to e^+e^-\gamma\gamma^* \to e^+e^-\eta^\prime$ using a single-tag technique, where only one scattered lepton is detected. The measurement covers $Q^2 \in [0.1, 6.0]$ GeV$^2$, achieving unprecedented precision, better than $3.0\%$ for $Q^2 < 1.5$ GeV$^2$, and providing the first direct determination at $Q^2 < 0.3$ GeV$^2$.

hep-ex↗
arXiv2026

Search for the charged lepton flavour violating decay $\eta'\to e\mu$

Based on $(8998\pm40)\times10^6$ $J/\psi$ events collected in $e^+e^-$ collisions at $\sqrt{s} = 3.097$ GeV with the BESIII detector, we present a search for the charged lepton flavour violating decay $\eta'\to e\mu$ with $J/\psi\to\gamma\eta'$. No significant signal is observed, and an upper limit on its decay branching fraction is set to be $6.3\times10^{-7}$ at the 90% confidence level, improving the previous best result by nearly three orders of magnitude.

hep-ex↗
arXiv2026

Search for the $\boldsymbol{B^0 \to K^0_{\rm S} \tau^+ \tau^-}$ decay

We present the first search for $B^0 \to K^0_{\rm S} \tau^+\tau^-$ decays. We look for signal decays in $B^0\bar B^0$ events produced in asymmetric-energy electron-positron collisions. This work uses samples from the Belle and Belle~II detectors, comprising 1.16 billion $\Upsilon(4S)$ events. In $\Upsilon(4S)\to B^0\bar{B}^0$ decays, the non-signal $\bar{B}^0$ meson is fully reconstructed in a hadronic channel. For the signal $B^0$ meson, $\tau$-lepton decays into final states with a single charged particle are selected. A multivariate classifier is used to combine several discriminating inputs into a single fit observable. We observe no evidence for the signal and set an upper limit on the branching fraction $\mathcal{B}(B^0\to K^0_{\rm S} \tau^+\tau^-) < 8.3 \times 10^{-4}$ at the 90\% confidence level. Combining this with the recent measurement of the isospin-partner decay $B^+\to K^+\tau^+\tau^-$, we determine an upper limit $\mathcal{B}(B\to K\tau^+\tau^-) < 5.4\times10^{-4}$ at the 90\% confidence level.

hep-ex↗
arXiv2026

Precision measurements of semleptonic decays $D^0 \to \pi^-\ell^+\nu_\ell$ and $D^+ \to \pi^0\ell^+\nu_\ell$ ($\ell =e,\mu$)

The branching fractions of $D^0\to \pi^-e^+\nu_e$, $D^0\to \pi^-\mu^+\nu_\mu$, $D^+\to \pi^0e^+\nu_e$, and $D^+\to \pi^0\mu^+\nu_\mu$ are measured to be $(2.950\pm0.017_{\rm stat.}\pm 0.017_{\rm syst.})\times10^{-3}$, $(2.817\pm0.037_{\rm stat.}\pm 0.019_{\rm syst.})\times10^{-3}$, $(3.622\pm0.034_{\rm stat.}\pm 0.018_{\rm syst.})\times10^{-3}$, and $(3.507\pm0.043_{\rm stat.}\pm 0.026_{\rm syst.})\times10^{-3}$ using $e^+e^-$ collision data with an integrated luminosity of 20.3 fb$^{-1}$ collected at the center-of-mass energy of 3.773 GeV with the BESIII detector. The partial decay rates of these four decays are measured with the best precision to date and their forward-backward asymmetries are determined for the first time. By performing a simultaneous fit to these results, the product of the hadronic transition form factor $f^{D\to\pi}_+(0)$ and the modulus of the $c\to d$ Cabibbo-Kobayashi-Maskawa matrix element $|V_{cd}|$ is given by $f^{D\to\pi}_+(0)|V_{cd}|=0.1425\pm0.0005_{\rm stat.}\pm0.0003_{\rm syst.}$. Taking the $|V_{cd}|$ provided by the standard model global fit and the $f^{D\to\pi}_+(0)$ calculated from the lattice quantum chromodynamics as input, we obtain $f^{D\to\pi}_+(0)=0.6339\pm0.0024_{\rm stat.}\pm0.0014_{\rm syst.}$ and $|V_{cd}|=0.2262\pm0.0008_{\rm stat.}\pm0.0005_{\rm syst.}\pm0.0018_{\rm LQCD.}$, respectively. The reported results have the best precision to date. We also search for the scalar current contribution in the $c\to d \ell^+\nu_{\ell}$ transition and determine Re$(C_S^\mu)=$ $0.022 \pm 0.023_{\rm stat.}\pm 0.003_{\rm syst.}$ and $|{\rm Im}(C_S^\mu)|=0.000 \pm $ $0.038_{\rm stat.} \pm 0.012_{\rm syst.}$. In addition, the lepton flavor universality is tested with the ratios of the decay rates between semimuonic and semielectronic decays in full and several $\ell^+\nu_\ell$ four-momentum transfer ranges.

hep-ex↗
arXiv2026

Precision Measurement of Decay Dynamics in $D^{0(+)}\to \pi^{-(0)}\ell^+\nu_\ell$

The branching fractions of $D^0\to \pi^-e^+\nu_e$, $D^0\to \pi^-\mu^+\nu_\mu$, $D^+\to \pi^0e^+\nu_e$, and $D^+\to \pi^0\mu^+\nu_\mu$ are precisely measured, using 20.3 fb$^{-1}$ of $e^+e^-$ collision data collected at the center-of-mass energy of 3.773 GeV with the BESIII detector. The ratios of the decay widths between muon and positron channels are examined in full, across several four-momentum transfer ranges of $\ell^+\nu_{\ell}$. No lepton flavor universality violation is found in the current data. From a simultaneous fit to the precisely measured partial decay rates and the first measured forward-backward asymmetries of these four decays, the product of the hadronic transition form factor, $f^{D\to\pi}_+(0)$, and the modulus of the $c\to d$ quark mixing element, $|V_{cd}|$, is measured with unprecedented precision to be $f^{D\to\pi}_+(0)|V_{cd}|=0.1425\pm0.0005_{\rm stat.}\pm0.0003_{\rm syst.}$. Taking the value of $|V_{cd}|$ from the standard model global fit and $f^{D\to\pi}_+(0)$ derived by the lattice quantum chromodynamics calculation as input, we obtain $f^{D\to\pi}_+(0)=0.1425\pm0.0005_{\rm stat.}\pm0.0003_{\rm syst.}$ and $|V_{cd}|=0.2262\pm0.0008_{\rm stat.}\pm0.0005_{\rm syst.}\pm0.0018_{\rm LQCD.}$, respectively. The precision of each result is a factor of 2-3 better than the previous best measurements. Additionally, the real and imaginary parts of the scalar current contribution in the $c\to d \ell^+\nu_{\ell}$ transition are measured for the first time to be Re $(C_S^\mu)=$ $0.022 \pm 0.023_{\rm stat.}\pm 0.003_{\rm syst.}$ and $|\mathrm{Im} (C_S^\mu)|=0.000 \pm 0.038_{\rm stat.}\pm 0.012_{\rm syst.}$.

hep-ex↗
arXiv2026

Multibranched parametric resonance and swallowtail catastrophe in electromechanical oscillators with nonlinear friction

Parametric resonance underpins the operation of a wide range of physical systems, from nanomechanical resonators to quantum-information systems and Ising machines. As an archetypal class of driven-dissipative systems, parametric oscillators are generally expected to exhibit a single pair of stable period-two states with opposite phases. This bistable behavior enables both the simulation of spin Hamiltonians and the preparation of superconducting cat states. Whether multiple pairs of such states can coexist in a single oscillator, however, remains an open question. Here, we show experimentally and theoretically that conventional controlled nonlinear friction can induce the coexistence of two distinct pairs of period-two states in a micromechanical oscillator. The friction is implemented via a canonical approach, utilizing a drive-induced resonant coupling that transfers two vibrational quanta from the oscillatory mode to a faster decaying mode. We demonstrate that the onset of multistability is governed by a swallowtail catastrophe and quantitatively map the associated bifurcation structure. Our results broaden the understanding of parametric resonance and establish micro- and nano-mechanical oscillators as a versatile platform for studying catastrophe theory and multistable nonequilibrium dynamics.

physics.class-ph↗
arXiv2026

Measurement of Born Cross Section for $e^+e^-\to K_S^0\bar{\Xi}^+\Sigma^-+\rm{c.c.}$ at $\sqrt{s} = 3.51-4.95$ GeV

Using $e^+e^-$ collision data collected with the BESIII detector at the BEPCII collider corresponding to a total integrated luminosity of 44~fb$^{-1}$, we present the first measurement of the Born cross sections for the process $e^+e^-\to K_S^0\bar{\Xi}^+\Sigma^-+\rm{c.c.}$ at 56 center-of-mass energies from 3.510 to 4.951~GeV. By fitting the dressed cross sections of $e^+e^-\to K_S^0\bar{\Xi}^+\Sigma^-+\rm{c.c.}$ with the assumption of a power-law function plus a charmonium(-like) resonance, i.e. $\psi(3770)$, $\psi(4040)$, $\psi(4160)$, $Y(4230)$, $Y(4360)$, $\psi(4415)$, {\it Y}(4500), $Y(4660)$, and {\it Y}(4710), no significant signal of any charmonium(-like) state decaying into the $K_S^0\bar{\Xi}^+\Sigma^-+\rm{c.c.}$ is observed. Upper limits on the product of the electronic width and branching fraction at the 90\% confidence level are given for each resonance. Combining this result with the previous measurement of the isospin-symmetric process $e^+e^-\to K^{-} \bar{\Xi}^{+} \Sigma^{0} + \rm{c.c.}$, the ratio of the Born cross sections, $R=\sigma^{B}(e^+e^-\to K_S^0\bar{\Xi}^+\Sigma^-+\rm{c.c.})/$$\sigma^{B}(e^+e^-\to K^-\bar{\Xi}^+\Sigma^0+\rm{c.c.})$, is found to be approximately 1.

hep-ex↗
arXiv2026

Proof of principle for nucleon polarization measurement at BESIII

A novel technique for measuring the spin polarization of final-state nucleons in a general-purpose spectrometer is validated. Using $10.09\times10^{9}$ $J/\psi$ events at BESIII, the asymmetry of polarized proton scattering on detector support material is measured, and is consistent with the expected value. This proves that a general-purpose spectrometer can be utilized as a large-acceptance polarimeter, providing the spin polarization in addition to the conventional four-momentum information of the final-state particles. With this technique, physics capabilities are enhanced for existing and future facilities in particle and nuclear physics.

hep-ex↗
arXiv2026

First Measurement of the Relative Phase between Proton Psionic Form Factors

The relative phase between the time-like form factors of the proton is a crucial observable for a complete understanding of its internal structure, yet it has remained unmeasured due to the formidable experimental challenge of determining the final-state polarization or having available polarized beams. With a novel technique that measures polarization via secondary scattering on spectrometer material, we use $10.09\times10^{9}$ $J/\psi$ events collected at BESIII to analyze the reaction $e^+e^-\rightarrow J/\psi \rightarrow p\bar{p}$. This allows the first determination of the sine of the relative phase between the proton psionic form factors, $\sin\Delta\Phi=-0.20\pm0.34_{\textrm{stat}}\pm0.11_{\textrm{syst}}$. This result provides the first direct insight into the complex dynamics of proton formation, and offers valuable new information to constrain theoretical models of nucleon structure.

hep-ex↗
arXiv2026

Observation of $\eta_{c} \to p\bar{p}\eta$ via $\psi(3686) \to \gamma p\bar{p}\eta$

The decay $\eta_c\to p\bar{p}\eta$ is observed for the first time with a significance of exceeding $10\sigma$. It is found by analyzing $(2712.4 \pm 14.3)\times10^{6}$ $\psi(3686)$ events accumulated at the BESIII detector. The measured branching fraction of $\eta_c\to p\bar{p}\eta$ via $\psi(3686) \to \gamma p \bar{p} \eta$ is significantly influenced by the interference between the resonant $\eta_c$ decay and the non-resonant process $\psi(3686) \to \gamma p \bar{p} \eta$ and is measured in both constructive- and destructive-interference scenarios. The joint branching fraction of $\psi(3686)\to \gamma\eta_c$, $\eta_c\to p\bar{p}\eta$ is measured to be $(3.2 \pm 0.1 \pm 0.9)\times10^{-6}$ or $(8.7 \pm 0.3 \pm 2.1)\times10^{-6}$ for constructive- or destructive-interference solutions, respectively, where the first uncertainties are statistical and the second systematic. The branching fraction of $\eta_c\to p\bar{p}\eta$ is determined to be $\mathcal{B}(\eta_c\to p\bar{p}\eta)=(0.90 \pm 0.04 \pm 0.21 \pm 0.13)\times10^{-3}$ or $(2.42 \pm 0.07 \pm 0.48 \pm 0.34)\times10^{-3}$ for the two solutions, respectively, where the third uncertainties are due to the uncertainty in the branching fraction of $\psi(3686)\to \gamma \eta_c$.

hep-ex↗
arXiv2026

Search for an isoscalar partner of the $Z_c(3900)$ in $e^+e^-\to\pi^+\pi^-\eta J/\psi$

Using a data sample corresponding to an integrated luminosity of 20.3 fb$^{-1}$ collected at center-of-mass energies from 4.18 to 4.95 GeV with the BESIII detector, we observe the process $e^+e^-\to\pi^+\pi^-\eta J/\psi$ with a statistical significance of $6.0 \sigma$, including systematic uncertainties. The isoscalar partner of the $Z_c(3900)$, denoted $X(3900)$, is searched for in the $\eta J/\psi$ final state, and no significant signal is observed. The upper limits on the product of the Born cross section $\sigma^{\rm Born}[e^{+}e^{-}\to\pi^{+}\pi^{-} X(3900)$] and the branching fraction $\mathcal{B}[X(3900)\to\eta J/\psi]$ are given with various assumptions for the mass and width of the $X(3900)$.

hep-ex↗