Enhanced production of charged over neutral kaons in Ar+Sc collisions measured by NA61/SHINE at CERN SPS
Tomasz Matulewicz
TL;DR
This study tests isospin symmetry in kaon production in isospin-symmetric Ar+Sc collisions at $\sqrt{s_{NN}}=11.9$ GeV by measuring double-differential yields of $K^+$, $K^-$, and $K_S^0$ (via $K_S^0\to\pi^+\pi^-$) and defining the ratio $R_K=\frac{\langle K^+\rangle+\langle K^-\rangle}{\langle K^0\rangle+\langle\overline{K}^0\rangle}=\frac{\langle K^+\rangle+\langle K^-\rangle}{2\langle K^0_S\rangle}$. The results show $R_K>1$ with a $3\sigma$ significance, challenging current hadron-production models, including HRG with isospin-violating effects, and motivate further measurements in other isospin-symmetric systems to map the effect and its energy dependence.
Abstract
The isospin symmetry, originating from similar masses of $u$ and $d$ quarks, if exact would result in equal numbers of charged ($K^+$ and $K^{-}$) and neutral ($K^0$ and $\overline{K}^0$) mesons produced in collisions of isospin-symmetric atomic nuclei. The charged and neutral $K$ meson production in Ar+Sc collisions at a center-of-mass energy of 11.9 GeV per nucleon pair was measured by the NA61/SHINE Collaboration. The production of charged $K$ mesons at mid-rapidity is (18.4$\pm$6.1)\% higher than that of the neutral $K$ mesons.The models of hadron production, including known isospin-symmetry breaking effects, cannot explain the measurements.