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

TL;DR

This paper measures the e^+e^- to pi^+pi^-J/psi cross section using initial state radiation at Belle, revealing a peak at 4.25 GeV/c^2 and an additional structure around 4.05 GeV/c^2, indicating possible new resonances.

Contribution

First measurement of the cross section in this energy range with detailed resonance structure analysis using interference fits.

Findings

Observation of a peak near 4.25 GeV/c^2 (Y(4260))

Identification of an additional structure around 4.05 GeV/c^2

Better fit with two interfering resonances than one

Abstract

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.