Study of $e^{+}{e}^{-}\to {\hat{I}}^3\ddot{I}‰J/\ddot{I}ˆ$ and Observation of $X(3872)\to \ddot{I}‰J/\ddot{I}ˆ$

Abstract

We study the $e^{+}{e}^{-}\to {\hat{I}}^3\ddot{I}‰J/\ddot{I}ˆ$ process using $11.6\mathrm{f}{\mathrm{b}}^{-1}$ $e^{+}{e}^{-}$ annihilation data taken at center-of-mass energies from $\sqrt{s}=4.008\mathrm{GeV}$ to $4.600\mathrm{GeV}$ with the BESIII detector at the BEPCII storage ring. The $X(3872)$ resonance is observed for the first time in the $\ddot{I}‰J/\ddot{I}ˆ$ system with a significance of more than $5\ddot{I}ƒ$. The relative decay ratio of $X(3872)\to \ddot{I}‰J/\ddot{I}ˆ$ and $\ddot{I}{€}^{+}\ddot{I}{€}^{-}J/\ddot{I}ˆ$ is measured to be $\mathcal{R}=1.6_{-0.3}^{+0.4}\pm 0.2$, where the first error is statistical and the second systematic (the same hereafter). The $\sqrt{s}$-dependent cross section of $e^{+}{e}^{-}\to {\hat{I}}^{3}X(3872)$ is also measured and investigated, and it can be described by a single Breit-Wigner resonance, referred to as the $Y(4200)$, with a mass of $4200.6_{-13.3}^{+7.9}\pm 3.0\mathrm{MeV}/c^2$ and a width of $115_{-26}^{+38}\pm 12\mathrm{MeV}$. In addition, to describe the $\ddot{I}‰J/\ddot{I}ˆ$ mass distribution above $3.9GeV/{\mathrm{c}}^2$, we need at least one additional Breit-Wigner resonance, labeled as $X(3915)$, in the fit. The mass and width of the $X(3915)$ are measured to be $3926.4\pm 2.2\pm 1.2MeV/{\mathrm{c}}^2$ and $3.8\pm 7.5\pm 2.6\mathrm{MeV}$, or $3932.6\pm 8.7\pm 4.7MeV/{\mathrm{c}}^2$ and $59.7\pm 15.5\pm 3.7\mathrm{MeV}$, respectively, depending on the fit models. The resonant parameters of the $X(3915)$ agree with those of the $Y(3940)$ in $B\to K\ddot{I}‰J/\ddot{I}ˆ$ and of the $X(3915)$ in ${\hat{I}}^3{\hat{I}}^3\to \ddot{I}‰J/\ddot{I}ˆ$ by the Belle and BABAR experiments within errors.