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(-) -> Lambda(+)(c)Lambda(-)(c) cross section as a function of center-of-mass energy near the Lambda(+)(c)Lambda(-)(c) threshold. A clear peak with a significance of 8.2 sigma is observed in the Lambda(+)(c)Lambda(-)(c) 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/c(2) and Gamma(tot) = [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 Upsilon(4S) resonance and nearby continuum with an integrated luminosity of 695 fb(-1) at the KEKB asymmetric-energy e(+)e(-) collider.