Evidence of isospin-symmetry violation in high-energy collisions of atomic nuclei

Strong interactions preserve an approximate isospin symmetry between up (u) and down (d) quarks, part of the more general flavor symmetry. In the case of K meson production, if this isospin symmetry were exact, it would result in equal numbers of charged (K+ and K−) and neutral (K0 and K0bar) mesons produced in collisions of isospin-symmetric atomic nuclei. In this talk, I will present recent experimental evidence of isospin-symmetry violation in high-energy argon-scandium collisions at √sₙₙ = 11.9A GeV, as observed by the NA61/SHINE collaboration. The measured K⁺, K⁻, and Kₛ⁰ yields at mid-rapidity show an unexpected ~18% excess of charged over neutral kaons, significantly exceeding model predictions based on isospin symmetry and neutron-proton asymmetry. The experimental setup, measurement methods, theoretical models, and implications of this discrepancy will be discussed.