The first experimental results on the polarization of inclusively produced Λ-hyperons in the 𝐾⁻- and 𝜋⁻-beams with a momentum of 26.5 GeV/𝑐 were obtained  at the SPASCHARM facility at the U-70 accelerator complex in NRC “Kurchatov institute” – IHEP (Protvino). The polarization of Λ-hyperons in the 𝜋⁻-beam does not exceed several percent in most of the studied kinematic region, with the exception of the region 𝑝_𝑇>1 GeV/𝑐, where the polarization is 23±9%. For 𝐾^—mesons incident on nuclear targets, in the region of large values ​​of the Feynman variable 𝑥_𝐹 and the transverse momentum 𝑝_𝑇, a substantially larger positive polarization was observed for the first. In the region 𝑝_𝑇>0.3 GeV/𝑐 the average polarization P_N = is 23.6 ± 3.6%, which is a 6.5 sigma effect. In the region 𝑝_𝑇>1 GeV/𝑐 polarization reaches 66 ±18%.

The observed spin effects may have a non-perturbative nature, associated with spontaneous breaking of chiral symmetry in QCD with formation of constituent quarks with mass of about 300 MeV and a large negative (-0.4) anomalous chromomagnetic moment, which could polarize quarks via the Stern-Gerlach effect in a non-uniform chromomagnetic fields. 

Dependence of 𝑃_𝑁 on the transverse momentum 𝑝_𝑇 in the reactions 𝜋⁻𝐴 → Λ𝑋 and 𝐾⁻𝐴 → Λ𝑋, obtained at an energy of 26.5 GeV at the SPASCHARM experimental facility at the U-70 accelerator complex in Protvino.

V.V. Abramov, V.V. Moiseev, I.G. Alekseev, N.A. Bazhanov et al.
JETP Letters 120, issue 6 (2024)