Observation of a Charged Charmoniumlike Structure in e+e-→π+π-J/ψ at s=4.26GeV

We study the process e⁺e^-→π⁺π^-J/ψ at a center-of-mass energy of 4.260 GeV using a 525pb^-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±1.9±3.7)pb, consistent with the production of the Y(4260). We observe a structure at around 3.9GeV/c² in the π^±J/ψ mass spectrum, which we refer to as the Z_c(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the π^±J/ψ invariant mass spectrum, neglecting interference, results in a mass of (3899.0±3.6±4.9)MeV/c² and a width of (46±10±20)MeV. Its production ratio is measured to be R=(σ(e⁺e^-→π^±Z_c(3900)^∓→π⁺π^-J/ψ)/σ(e⁺e^-→π⁺π^-J/ψ))=(21.5±3.3±7.5)%. In all measurements the first errors are statistical and the second are systematic.