Observation of a near-threshold enhancement in the e+e- →Λc+Λc- Cross section using initial-state radiation

We report a measurement of the exclusive e+e-→Λc+Λc- cross section as a function of center-of-mass energy near the Λc+Λc- threshold. A clear peak with a significance of 8.2σ is observed in the Λc+Λc- invariant mass distribution just above threshold. With an assumption of a resonance origin for the observed peak, a mass and width of M=[4634-7+8(stat)-8+5(syst)]MeV/c2 and Γtot=[92- 24+40(stat)-21+10(syst)]MeV are determined. The analysis is based on a study of events with initial-state-radiation photons in a data sample collected with the Belle detector at the Υ(4S) resonance and nearby continuum with an integrated luminosity of 695fb-1 at the KEKB asymmetric-energy e+e- collider. © 2008 The American Physical Society.