Collisions of atomic nuclei at relativistic velocities produce new particles, predominantly mesons containing one valence quark and one valence anti-quark. These particles are produced in strong interactions, which preserve an approximate symmetry between up ($u$) and down ($d$) quarks. In the case of $K$ meson production, if this symmetry were exact, it would result in equal numbers of charged ($K^+$ and $K^-$) and neutral ($K^0$ and $\overline K^0$) mesons in the final state. In this Letter, we report a measurement of the relative abundance of charged over neutral $K$ meson production in collisions of argon and scandium nuclei at a center-of-mass energy of 11.9~GeV per nucleon pair. We find that production of $\mathit{K^+}$ and $\mathit{K^-}$ mesons at mid-rapidity displays a significant excess of $(23.3\pm 5.5)\%$ relative to that of the neutral $K$ mesons. The origin of this unexpected excess remains to be elucidated.
Comment: 18 pages, 6 figures