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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 anapproximate 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 numbersof charged ($K^+$ and $K^-$) and neutral ($K^0$ and $\overline{K}$$^0$) mesonsin the final state. In this Letter, we report a measurement of the relativeabundance of charged over neutral $K$ meson production in collisions of argonand scandium nuclei at a center-of-mass energy of 11.9~GeV per nucleon pair. Wefind that production of $K^+$ and $K^-$ mesons at mid-rapidity displays asignificant excess of $(23.3\pm 5.7)\%$ relative to that of the neutral $K$mesons. The origin of this unexpected excess remains to be elucidated.

Excess of charged over neutral $K$ meson production in high-energy collisions of atomic nuclei