Observation of a threshold enhancement in the p \bar{Lambda} invariant mass spectrum

Abstract

An enhancement near the m_p + M_Λ mass threshold is observed in the combined p \barΛ and \bar{p}Λinvariant mass spectrum from J/ψ--> p K^- \bar{Lambda} + c.c. decays. It can be fit with an S-wave Breit-Wigner resonance with a mass m = 2075 +- 12 (stat) +- 5 (syst) MeV and a width of Γ= 90 +- 35 (stat) +- 9 (syst) MeV; it can also be fit with a P-wave Breit-Wigner resonance. Evidence for a similar enhancement is also observed in ψ' --> p K^- \barΛ + c.c. decays. The analysis is based on samples of 5.8 X 10^7 J/ψand 1.4 X 10^7 ψ' decays accumulated in the BES II detector at the Beijing Electron-Positron Collider.