Kaon, anti-kaon and phi production in near-threshold heavy-ion collisions

Kaons and anti-kaons are sensible to the potential which they experince in the course of heavy-ion collisions. In particular, the azimuthal anisotropy, often termed squeeze-out pattern, depends sensitively on the interplay of the scalar and vector components of the interaction. Due to this interaction the kaons and anti-kaons behave quite differently in a hadron medium: anti-kaons display a remarkable effective mass reduction with increasing density, while the kaons have a slightly increased effective mass. Indeed, the  KaoS  collaboration at GSI Darmstadt found strong evidences for an anti-kaon mass reduction: at equivalent beam energies the kaon and anti-kaon production cross sections become equal (in contrast, in elementary pp reactions slightly above threshold they differ by a factor 10 or more). Other findings corroborate this effect. For example, the ration of K- to K+ multiplicities increase with decreasing center-of-mass kinetic energy.

Via the dominant K+ K- decay channel the phi production in heavy-ion collisions can be investigated. The  FOPI detector at GSI Darmstadt is well suited for such investigations. First data analyses of the reactions Ru + Ru at 1.69 A GeV point to a j/Ki- multiplicity ratio of 0.5 at mid-rapidities and low transverse momenta. In such a way the systematic understanding of the strangeness dynamics is improved.