An improved method for determining the electric dipole moment of Λ hyperon

A CP-odd observable has been proposed by He and collaborators to test the CP conservation in J / ψ → Λ over(Λ, ̄) and its expectation value can be used to determine the electric dipole moment of Λ [X.G. He, J.P. Ma, B. McKellar, Phys. Rev. D 47 (1993) R1744; X.G. He, J.P. Ma, B. McKellar, Phys. Rev. D 49 (1994) 4548]. Instead of measuring the expectation value of the CP-odd observable directly, we proposed an indirect method which can significantly improve the statistical sensitivity. With this improved method, Monte Carlo studies show that an expected upper limit of 0.4 × 10^{- 16} e cm on the electric dipole moment of Λ can be obtained based on 10⁴ fully reconstructed J / ψ → Λ over(Λ, ̄), Λ → p π^-, over(Λ, ̄) → over(p, ̄) π⁺ events. This expected upper limit is about a factor 4 smaller than the one obtained from the direct experimental search to date. The result suggests an unprecedented experimental sensitivity on detecting the electric dipole moment of Λ could be reached even with the data sample already collected by BESII.