Measurement of Λ Polarization in the π− p → K 0Λ Reaction at p_π ⁻=1.33 GeV/ c toward a New Λ p Scattering Experiment

Understanding the spectrum and dynamics of excited nucleon states (N* resonances) remains a central challenge in hadron physics, as these resonances emerge in intermediate states of πN and γN interactions. The dynamical coupled-channel (DCC) models addressing this problem rely critically on experimental inputs for partial wave analyses (PWA). However, polarization observables of hyperons, which serve as sensitive probes of reaction mechanisms, have so far been limited in precision. To end this, we aim to provide high-precision measurements of the polarization of the Λ hyperon produced in the π−p → K0Λ reaction. The data were obtained with the J-PARC E40 experiment at the K1.8 beamline of the J-PARC Hadron Experimental Facility. The polarization of the Λ was precisely measured in the angular range 0.6 < cos θ K0CM< 1.0 with a fine bin width of dcos θ K0CM= 0.05. The observed average polarization in the region 0.60 < cos θ K0CM< 0.85 was 0.932 ± 0.058(stat) ± 0.028(syst). These results provide essential input for PWAs of DCC models, contributing to a deeper understanding of N* resonance dynamics. Furthermore, the strong Λ polarization observed indicates the feasibility of developing a highly polarized Λ beam, opening prospects for future Λp scattering experiments such as J-PARC E86.