Isospin kaon anomaly and its consequences

Abstract 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 $$6.4\sigma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>6.4</mml:mn> <mml:mi>σ</mml:mi> </mml:mrow> </mml:math> –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 $$p/n \sim 1.2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>p</mml:mi> <mml:mo>/</mml:mo> <mml:mi>n</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>1.2</mml:mn> </mml:mrow> </mml:math> and the hyperon ratio $$\Sigma ^{+}/\Sigma ^{-}\sim 1.4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>Σ</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>/</mml:mo> <mml:msup> <mml:mi>Σ</mml:mi> <mml:mo>-</mml:mo> </mml:msup> <mml:mo>∼</mml:mo> <mml:mn>1.4</mml:mn> </mml:mrow> </mml:math> . Finally, we extend the approach to antiquarks in the initial state, useful for e.g. pion–nucleus scattering reactions.