Kaon squeeze-out in heavy ion reactions

Abstract

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.

Kaon production in heavy ion reactions at intermediate energies

“…First data from KaoS [161] for semi-central Au+Au at 1 AGeV showed a clear squeezeout signal for midrapidity kaons. In corresponding transport calculations from the Texas/Stony Brook group [217,161] and the Tübingen group [179] the data could only be reproduced by the presence of the repulsive K + mean field. Elastic rescattering of K + mesons was found to be too weak to create the observed squeeze-out signal.…”

“…The first predictions for K − out-of-plane emission pattern were made by Wang et al [179] and are shown in Fig. 36.…”

“…Here and in the previous works of the Tübingen group [46,109,123,159,179,180,181,182] for pion induced reactions the elementary cross sections of Tsushima et al [110] have been used. The cross sections are derived within the resonance model in Born approximation, including all baryonic resonances with masses below 2 GeV as intermediate states.…”

Review of the results of the KaoS Collaboration

“…A large fraction of the observed out-of-plane enhancement, in contrast to other 5) with IQMD model calculations [22]. models [23,24,25], is caused by the scattering of K + mesons. The transport code (HSD) [25] in contrast predicts a dominant influence of the potential on the emission pattern of the K + mesons.…”

“…The transport code (HSD) [25] in contrast predicts a dominant influence of the potential on the emission pattern of the K + mesons. In the system Au+Au at 1 AGeV where both size and life time of the fireball are larger than in the Ni+Ni case, the effect of the repulsive K + N potential was found to be very important [8,23,24].…”