Measurement of thee+e−→π+π−J/ψCross Section Via Initial-State Radiation at Belle

The cross section for e{sup +}e{sup -}{yields}{pi}{sup +}{pi}{sup -}J/{psi} between 3.8 and 5.5 GeV/c{sup 2} is measured using a 548 fb{sup -1} data sample collected on or near the {upsilon}(4S) resonance with the Belle detector at KEKB. A peak near 4.25 GeV/c{sup 2}, corresponding to the so called Y(4260), is observed. In addition, there is another cluster of events at around 4.05 GeV/c{sup 2}. A fit using two interfering Breit-Wigner shapes describes the data better than one that uses only the Y(4260), especially for the lower-mass side of the 4.25 GeV enhancement.