Observation of a Broad Structure in the π+π−J/ψ Mass Spectrum around 4.26 GeV/c2

We study initial-state radiation events, e + e − → γ ISR π + π − J / ψ , with data collected with the BABAR detector. We observe an accumulation of events near 4.26     GeV / c 2 in the invariant-mass spectrum of π + π − J / ψ . Fits to the mass spectrum indicate that a broad resonance with a mass of about 4.26     GeV / c 2 is required to describe the observed structure. The presence of additional narrow resonances cannot be excluded. The fitted width of the broad resonance is 50 to 90     MeV / c 2 , depending on the fit hypothesis.