Meson production in dense nuclear matter

Pion and Kaon production has been studied in symmetric nucleus-nucleus collisions at beam energies between 0.8 and 1.8 AGeV. The anisotropic azimuthal emission of pions in semi-central collisions can be explained by the emission of high-energy pions in an early stage of the collision and a late "freeze-out" of low-energy pions. In nucleus-nucleus collisions at I AGeV, the K+ multiplicity increases more than linearly both with A (in A + A collisions) and with Apart (i.e. the number of participating nucleons in Au+Au collisions). This nonlinear behaviour is due to collective effects such as multiple hadron-hadron encounters. According to transport calculations, the large K+ cross section observed for Au+Au collisions at 1 AGeV is a consequence of a soft nuclear equation of state. The large K-/K+ ratio measured in Ni+Ni collisions at equivalent beam energies (compared to p+p collisions) is a signature for an enhanced in-medium K-production. In order to reproduce the data, transport models have to consider a reduction of the K- mass in the dense nuclear medium.