First measurement of antikaon phase-space distributions in nucleus–nucleus collisions at subthreshold beam energies

Differential production cross sections of K− and K+ mesons have been measured as function of the polar emission angle in Ni + Ni collisions at a beam energy of 1.93 A GeV. In near-central collisions, the spectral shapes and the widths of the rapidity distributions of K− and K+ mesons are in agreement with the assumption of isotropic emission. In non-central collisions, the K− and K+ rapidity distributions are broader than expected for a single thermal source. In this case, the polar angle distributions are strongly forward–backward peaked and the nonisotropic contribution to the total yield is about one third both for K+ and K− mesons. The K−/K+ ratio is found to be about 0.03 independent of the centrality of the reaction. This value is significantly larger than predicted by microscopic transport calculations if in-medium modifications of K mesons are neglected.