The collective flow of kaons and antikaons has been measured in centrality selected Au+Au and Ni+Ni collisions at beam energies between 1 and 2 AGeV with the Kaon Spectrometer at the GSI in Darmstadt. The investigation of the azimuthal angular emission pattern of K^+ and of K^- mesons provides a unique possibility to address the questions of the strangeness production mechanisms and of the in-medium Kaon-Nucleon potentials at SIS energies. The measured squeeze-out of K^+ in Au+Au collisions at 1 AGeV has been considered as an indication of the repulsive K^+N in-medium potential [1]. However, recent theoretical considerations attribute substantial part of the squeeze-out signal to re-scattering and shadowing effects [3]. On the other hand, the in-plane flow of antikaons in Ni+Ni collisions, observed for the first time at these energies, could be reproduced in the calculations taking into account an attractive K^-N potential [2].
The new high statistics measurement of K^+ and K^- mesons in Au+Au collisions at 1.5 AGeV close to mid-rapidity (0.2 < y/y_^{beam} < 0.6) allowed to determine the transverse momentum dependence (0.2 < p_t < 0.8 GeV/c) of their elliptic flow. For K^+ mesons the squeeze-out signal has been found to be independent of p_t. For K^- mesons a cross-over change in v_2 from weak in-plane flow at low pt to a clear out-of-plane signal at high pt has been observed which provides significant new constraints for the transport model descriptions[4].
[1] Y. Shin et al., Phys.Rev.Lett., 81:1576-1579 (1998)
[2] F. Uhlig et al., Phys. Rev. Lett., 95:012301 (2005).
[3] C. Hartnack and J. Aichelin, J. Phys. G 30(1) (2004).
W. Cassing et al., Nucl. Phys. A727 (2003).
E. Bratkovskaya, private communication (2005).
[4] KaoS collaboration, paper in preparation.