Using a data sample of ψ(3770) events collected with the BESIII detector at BEPCII corresponding to an integrated luminosity of 2.9fb−1, we report a measurement of Λ spin polarization in e+e−→ΛΛ¯ at s=3.773GeV. The significance of polarization is found to be 2σ including the systematic uncertainty, which implies a zero phase between the transition amplitudes of the ΛΛ¯ helicity states. This phase can be interpreted in terms of psionic form factors, and is determined to be ΔΦΨ=ΦEΨ−ΦMΨ=(71−46+66±5)°. Similarly, the ratio between the form factors is found to be Rψ=|GEΨ/GMΨ|=0.48−0.35+0.21±0.03. The first uncertainties are statistical and the second systematic.

• Received 23 November 2021
• Accepted 20 December 2021
• Corrected 14 January 2022

DOI:https://doi.org/10.1103/PhysRevD.105.L011101

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Published by the American Physical Society