Observation of a charged charmoniumlike structure in e^+e^-\rightarrow\pi^+\pi^-J\psi at \sqrt{s}=4.26 GeV
M. Ablikim et al.
Accepted
We study the process $\EE\to \pcpcjpsi$ at a center-of-mass energy of $4.260$~GeV using a 525~pb$^{-1}$ data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be $(62.9\pm 1.9\pm 3.7)$~pb, consistent with the production of the $\y$. We observe a structure at around 3.9~GeV/$c^2$ in the $\pi^\pm \jpsi$ mass spectrum, which we refer to as the $\X$. If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the $\pi^\pm\jpsi$ invariant mass spectrum, neglecting interference, results in a mass of $(3899.0\pm 3.6\pm 4.9)~{\rm MeV}/c^2$ and a width of $(46\pm 10\pm 20)$~MeV. Its production ratio is measured to be $R=\frac{\sigma(\EE\to \pi^\pm \X^\mp\to \pcpcjpsi))} {\sigma(\EE\to \pcpcjpsi)}=(21.5\pm 3.3\pm 7.5)\%$. In all measurements the first errors are statistical and the second are systematic.