Observation of a charged charmoniumlike structure in e⁺e^- → π⁺π^-J/φ at √s=4.26 GeV

We study the process e⁺e^- → π⁺π^-J/φ 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 ± 1.9 ± 3.7) pb, consistent with the production of the Y(4260). We observe a structure at around 3.9 GeV=c² in the π^± J/φ 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/φ 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_c(3900)^± → π⁺π^-J/φ )) = (21.5 ± 3.3 ± 7.5)% In all measurements the first errors are statistical and the second are systematic.