# Study of ${e}^{+}{e}^{-}\to \gamma \omega J/\psi$ and Observation of $X\left(3872\right)\to \omega J/\psi$

18 June 2019

Abstract: We study the ${e}^{+}{e}^{-}\to \gamma \omega J/\psi$ process using $11.6\text{}\text{}{\mathrm{fb}}^{-1}$ ${e}^{+}{e}^{-}$ annihilation data taken at center-of-mass energies from $\sqrt{s}=4.008\text{}\text{}\mathrm{GeV}$ to 4.600 GeV with the BESIII detector at the BEPCII storage ring. The $X\left(3872\right)$ resonance is observed for the first time in the $\omega J/\psi$ system with a significance of more than $5\sigma$. The relative decay ratio of $X\left(3872\right)\to \omega J/\psi$ and ${\pi }^{+}{\pi }^{-}J/\psi$ is measured to be $R={1.6}_{-0.3}^{+0.4}±0.2$, where the first uncertainty is statistical and the second systematic (the same hereafter). The $\sqrt{s}$-dependent cross section of ${e}^{+}{e}^{-}\to \gamma X\left(3872\right)$ is also measured and investigated, and it can be described by a single Breit-Wigner resonance, referred to as the $Y\left(4200\right)$, with a mass of ${4200.6}_{-13.3}^{+7.9}±3.0\text{}\text{}\mathrm{MeV}/{c}^{2}$ and a width of ${115}_{-26}^{+38}±12\text{}\text{}\mathrm{MeV}$. In addition, to describe the $\omega J/\psi$ mass distribution above $3.9\text{}\text{}\mathrm{GeV}/{c}^{2}$, we need at least one additional Breit-Wigner resonance, labeled as $X\left(3915\right)$, in the fit. The mass and width of the $X\left(3915\right)$ are determined. The resonant parameters of the $X\left(3915\right)$ agree with those of the $Y\left(3940\right)$ in $B\to K\omega J/\psi$ and of the $X\left(3915\right)$ in $\gamma \gamma \to \omega J/\psi$ observed by the Belle and BABAR experiments within errors.

Published in: Physical Review Letters 122 (2019)