Isospin kaon anomaly and its consequences

Isospin symmetry is well fulfilled in the QCD vacuum, as evidenced by small mass differences of isospin partners and suppressed isospin-violating decays. Recently, the NA61/SHINE collaboration reported an unexpectedly large isospin-violating charged-to-neutral kaon ratio in Ar-Sc heavy-ion collisions (HIC). Using a quark recombination approach, we introduce a function of kaon multiplicities that reduces to unity in the isospin-symmetric limit independently of the scattering energy and type of nuclei. Using this quantity, we show that nucleus–nucleus collisions violate isospin sizably (at the \(6.4\sigma \)–level), while proton-proton data on kaon multiplicities do not. We predict other isospin-violating enhancements in HIC, such as the proton-to-neutron ratio \(p/n \sim 1.2\) and the hyperon ratio \(\Sigma ^{+}/\Sigma ^{-}\sim 1.4\). Finally, we extend the approach to antiquarks in the initial state, useful for e.g. pion–nucleus scattering reactions.