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 − ) / ( K 0 + 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 charged Kaons and neutral Kaons, related to the broken SU(3) flavor symmetry already in elementary reactions. Practically, we implement this asymmetry into the simulation by adjusting the relative up- and down-quark probabilities in the elementary color field fragmentation process.