Measurement of e^+e^- to pi^+pi^-J/psi Cross Section via Initial State Radiation at Belle

Understand

The cross section for e^+e^- to pi^+pi^-J/psi between 3.8 and 5.5 GeV/c^2 is measured using a 548 fb^{-1} data sample collected on or near the Upsilon(4S) resonance with the Belle detector at KEKB.

• A peak near 4.25 GeV/c^2, corresponding to the so called Y(4260), is observed.
• In addition, there is another cluster of events at around 4.05 GeV/c^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.