Azimuthal Emission Patterns of $K^{+}$ and of $ K^{-} $ Mesons in Ni +   Ni Collisions near the Strangeness Production Threshold
  Azimuthal emission patterns of $K^\pm$ mesons have been measured in Ni + Ni collisions with the FOPI spectrometer at a beam kinetic energy of 1.91 A GeV. The transverse momentum $p_{T}$ integrated directed and elliptic flow of $K^{+}$ and $K^{-}$ mesons as well as the centrality dependence of $p_{T}$ - differential directed flow of $K^{+}$ mesons are compared to the predictions of HSD and IQMD transport models. The data exhibits different propagation patterns of $K^{+}$ and $K^{-}$ mesons in the compressed and heated nuclear medium and favor the existence of a kaon-nucleon in-medium potential, repulsive for $K^{+}$ mesons and attractive for $K^{-}$ mesons. 
V. Zinyuk, T.I. Kang, Y. Leifels, N. Herrmann, B. Hong, R. Averbeck,   A. Andronic, V. Barret, Z. Basrak, N. Bastid, M.L. Be...
nucl-ex