Observation of the polarization of $\Lambda$ hyperons produced in the interaction of $K^-$ mesons with nuclei

First results on the polarization of $\Lambda $ hyperons inclusively produced on 26.5-GeV/c ${{K}^{ - }}$ and ${{\pi }^{ - }}$ beams are presented. The measurements were carried out using the SPASCHARM setup at the U-70 accelerator facility (Protvino, Russia) on nuclear targets in 2021 and 2022. The polarization of an $\Lambda $ hyperon on the ${{\pi }^{ - }}$ beam does not exceed several percent in most of the studied kinematic region. The data obtained on the ${{K}^{ - }}$ -meson beam exhibit a significant positive polarization at large values of the Feynman variable xF and transverse momentum pT measured for the first time on nuclei.