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 $6.4σ$--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 $Σ^{+}/Σ^{-}\sim1.4$. Finally, we extend the approach to antiquarks in the initial state, useful for e.g. pion-nucleus scattering reactions.

Figures (3)

• Figure 1: Experimental data for $\tilde{R}_K$ of Eq. (\ref{['rkt']}) from proton-proton collisions, together with the weighted average (constant fit) $\tilde{R}_K =1.030\pm0.038$. The dashed line is the isospin-symmetric prediction $\tilde{R}_K = 1$.
• Figure 2: Experimental data for $\tilde{R}_K$ of Eq. (\ref{['rkt']}) from nucleus-nucleus collisions, together with the weighted average (constant fit) $\tilde{R}_K=1.185\pm0.029$. The dashed line is the isospin-symmetric prediction $\tilde{R}_K = 1$.
• Figure 3: Experimental data for $R_K$ of Eq. (\ref{['rk1']}) from nucleus-nucleus collisions, together with the weighted average (constant fit) $R_K = 1.152 \pm 0.027$. The dashed line is the isospin-symmetric prediction $R_K = 1$ valid for $Q/A = 1/2$.