Observation of a threshold enhancement in the plambda invariant-mass spectrum.

An enhancement near the m_p + M_{\Lambda} mass threshold is observed in the combined p \bar{\Lambda} and \bar{p}\Lambda invariant mass spectrum from J/\psi --> 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 \Gamma = 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 \psi' --> p K^- \bar{\Lambda} + c.c. decays. The analysis is based on samples of 5.8 X 10^7 J/\psi and 1.4 X 10^7 \psi' decays accumulated in the BES II detector at the Beijing Electron-Positron Collider.