Measurement of J/ψ decays into Λ π+π−

Based on 58 million J/ψ events collected by the BES II detector at the BEPC, J/ψ → Λ π+π− is observed for the first time. The branching fraction is measured to be Br(J/ψ → Λ π+π−) = (4.30±0.13±0.99)×10−3, excluding the decays to intermediate states, namely J/ψ → Ξ−+, J/ψ → Σ(1385)−(1385)+, and J/ψ → Σ(1385)+(1385)−. The branching fractions for these intermediate resonance channels are measured to be: Br(J/ψ → Ξ−+) = (0.90±0.03±0.18)×10−3, Br(J/ψ → Σ(1385)−(1385)+) = (1.23±0.07±0.30)×10−3, and Br(J/ψ → Σ(1385)+(1385)−) = (1.50±0.08±0.38)×10−3, respectively. The angular distribution is of the form ∝ (1 + αcos2θ) with α = (0.35±0.29±0.06) for J/ψ → Ξ−+, α = (−0.54±0.22±0.10) for J/ψ → Σ(1385)−(1385)+, and α = (−0.35±0.25±0.06) for J/ψ → Σ(1385)+(1385)−.