Azimuthal Emission Patterns of K+ and of K- Mesons in Ni+Ni Collisions near the Strangeness Production Threshold

Azimuthal emission patterns of K ± 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 hadron string dynamics and isospin quantum molecular dynamics 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.