Explanation of the observed violation of isospin symmetry in relativistic nucleus-nucleus reactions

The violation of isospin symmetry in nucleus-nucleus reactions, as shown in the ratio $R_K=(K^{+}+K^{-})\mathrm{/}(K^0+{\bar{K}}^0)$ presented by NA61/SHINE, can be understood by introducing results from color-string fragmentation in $e^{+}{e}^{-}$ to nuclear reactions. This novel input allows for a consistent description of the $e^{+}{e}^{-}$ data, proton+proton data and finally nucleus-nucleus data at all investigated energies. We conclude that the observed isospin violation in nucleus-nucleus reactions is explained by asymmetric production of up- and down-quarks in the elementary color field fragmentation process.