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We study the process e+e- to pi+pi-J/psi at a center-of-mass energy of 4.260GeV using a 525 pb^{-1} data sample collected with the BESIII detectoroperating at the Beijing Electron Positron Collider. The Born cross section ismeasured to be (62.9\pm 1.9\pm 3.7) pb, consistent with the production of theY(4260). We observe a structure at around 3.9 GeV/c^2 in the \pi^\pm J/psi massspectrum, 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 \pi^\pm J/psi invariant mass spectrum, neglecting interference,results in a mass of (3899.0\pm 3.6\pm 4.9) MeV/c^2 and a width of (46\pm 10\pm20) MeV. Its production ratio is measured to be R=\frac{\sigma(e+e- to \pi^\pmZ_c(3900)^\mp to pi+pi-J/psi))} {\sigma(e+e- to pi+pi-J/psi)}=(21.5\pm 3.3\pm7.5)%. In all measurements the first errors are statistical and the second aresystematic.

Observation of a charged charmoniumlike structure in e+e- to pi+pi-J/psi at \sqrt{s}=4.26 GeV