Kaon squeeze-out in heavy ion reactions

The squeeze-out phenomenon of K+ and K- mesons, i.e. the azimuthal asymmetry of K+ and K- mesons emitted at midrapidity in heavy ion reactions, is investigated for beam energies of 1-2 A.GeV. It is found that the squeeze-out signal is strongly affected by in-medium potentials of these mesons. The repulsive K+ nucleus potential gives rise to a pronounced out-of-plane emission of K+'s at midrapidity. With the K+ potential we reproduce well the experimental data of the K+ azimuthal distribution. It is found that the attractive K--nucleus potential cancels to a large extent the influence of rescattering and reabsorption of the K- mesons on the projectile and target residuals (i.e. shadowing). This results in an azimuthally isotropic emission of the midrapidity K- mesons with transverse momentum up to 0.8 GeV/c. Since it is well accepted that the shadowing alone would lead to a significant out-of-plane preference of particle emission, in particular at high transverse momenta, the disappearance of the out-of-plane preference for the K- mesons can serve as an unambiguous signal of the attractive K- potential. We also apply a covariant formalism of the kaon dynamics to the squeeze-out phenomenon. Discrepancies between the theory and the experiments and possible solutions are discussed.