Observation of a Threshold Enhancement in thepΛ¯Invariant-Mass Spectrum

An enhancement near the $m_p+M_{\Lambda }$ mass threshold is observed in the combined $p\overline{\Lambda }$ and $\overline{p}\Lambda$ invariant-mass spectrum from $J/\psi \to p{K}^{-}\overline{\Lambda }+\mathrm{c}.\mathrm{c}.$ decays. It can be fit with an $S$-wave Breit-Wigner resonance with a mass $m=2075\pm 12(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 5(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{M}\mathrm{e}\mathrm{V}$ and a width of $\Gamma =90\pm 35(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 9(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mathrm{M}\mathrm{e}\mathrm{V}$; it can also be fit with a $P$-wave Breit-Wigner resonance. Evidence for a similar enhancement is also observed in ${\psi }^{\prime }\to p{K}^{-}\overline{\Lambda }+\mathrm{c}.\mathrm{c}.$ decays. The analysis is based on samples of $5.8\times {10}^{7}J/\psi$ and $1.4\times {10}^7{\psi }^{\prime }$ decays accumulated in the BES II detector at the Beijing Electron-Positron Collider.