First Observation of $Λ$ Hyperon Transverse Polarization in ψ(3686)→ΛΛ

Based on (448.1±2.9)×10⁶ $ψ(3686)$ events collected with the BESIII detector at the BEPCII collider, we present the first observation of spin transverse polarization of $Λ$ and Λ hyperons produced coherently in the decay ψ(3686)→Λ(→ pπ⁻)Λ(→ pπ⁺). The relative phase between the electric and magnetic hadronic form factors is measured to be ΔΦ=(21.0±3.7stat.±0.8syst.)∘. The angular distribution parameter α_ψ=0.83±0.02stat.±0.01syst. is determined with a precision improved by a factor of 3.7 compared to the previous measurement. The relative phase between the S- and D-wave amplitudes for ΛΛ is observed, and the effective interaction radius is determined to be 0.0450±0.0026stat.±0.0012syst. fm. These results provide new insights into the strong interaction mechanisms and the internal structure of baryons.