Study of e+e- → π+π- J/ψ and Observation of a Charged Charmoniumlike State at Belle

The cross section for e(+)e(-) -> pi(+)pi(-) J/psi between 3.8 and 5.5 GeV is measured with a 967 fb(-1) data sample collected by the Belle detector at or near the Upsilon(nS) (n = 1, 2, . . . , 5) resonances. The Y(4260) state is observed, and its resonance parameters are determined. In addition, an excess of pi(+)pi(-) J/psi production around 4 GeV is observed. This feature can be described by a Breit-Wigner parametrization with properties that are consistent with the Y(4008) state that was previously reported by Belle. In a study of Y(4260) -> pi(+)pi(-) J/psi decays, a structure is observed in the M(pi(+/-) J/psi) mass spectrum with 5.2 sigma significance, with mass M = (3894.5 +/- 6.6 +/- 4.5) MeV/c(2) and width Gamma = (63 +/- 24 +/- 26) MeV/c(2), where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmoniumlike state.