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In the framework of the relativistic quantum molecular dynamics approach we investigate antiproton (p) observables in Au+Au collisions at 10.78 GeV. The rapidity dependence of the in-p)ane directed transverse momentum p, (p ) of p's sho~s the opposite sign of the nucleon How, which has indeed recently been discovered at 10.7A GeV by the E877 group. The "antiAow" of p's is also predicted at 2A GeV and at 1603 GeV and appears at all energies also for z's and N 's. These predicted p anticorrelations are a direct proof of strong p annihilation in massive he;ivy ion reactions. PACS numbers: 25.75.+r Antibaryons (8) have a large annihilation cross section in baryon-rich environments formed in heavy ion reaction». By studying their production and absorption meeh-.ini»m» we hope to get information about the time evolution of the baryon density in the reaction region [1][2][3][4].In previous calculations [5] experimental antiproton (p) d.it;i in the energy regime of' the BNL Alternating Gr;idient Synchrotron (AGS) have been explained on a microscopic level. The final particle yields have been interpreted in terms of two counterbalancing efTects: On the one hand, B production is enhanced due to collective efrect»; on the other hand, B annihilation becomes stronger due to nuclear stopping and therefore the baryon density increases. The strength of these competing proce»ses depends strongly on the incident energy and the reaction volume.Recent measurements of inclusive p spectra with proton, silicon, and gold beams at the AGS (IO-15A GeV)[6-10] do not give clues about the strength of such opposite efrects. These resultswith an uncertainty of a factor 2 in the p yields as extrapolated from pp collisions are compatible with relativistic quantum molecular dynamics (RQMD) calculations as well as with the first colli»ion model: Antibaryons are produced similar to the first collision yields if the absorption is neglected. There are other theoretical calculations which predict that B are also enhanced in a quark-gluon plasma event [11], by chiral symmetry restoration [12], in-medium etTects [13]. or by string-string interactions [14].In this Letter we demonstrate that strong annihilation di»(ort» considerably the momentum distribution of antibaryon». Because of the buildup of a high density and pressure zone, the nucleons stemming from the projectile bounce ofT in the reaction plane just into the opposite direction of the target nucleons in noncentral collisions.Antibaryon»and also pions and negatively charged kaon»show as a result of rescattering and absorption a »trong "antiAow", i.e. , anticorrelations to the nucleons in the reaction plane. This leads to an observable asymmetry in the azimuthal angular distributions (dJV"-/dttt) or in the mean directed transverse momentum of antiproton» in the reaction plane as a function of the rapidity p"(i ). This can be experimentally tested once the reaction plane i» determined, e. g., by measuring the azimuthal distribution of forward and backward going baryons or transverse energy in the ...

In the framework of RQMD we investigate antiproton observables in massive heavy ion collisions at AGS energies and compare to preliminary results of the E878 collaboration. We focus here on the considerable influence of the real part of an antinucleon-nucleus optical potential on thep momentum spectra.PACS numbers: 14.20 Dh, 25.70.-z Antibaryon production is a promising observable for collective effects in nucleus-nucleus collisions. On the other hand, N 's suffer strong final-state interactions. These interactions have two components which can be related to the N self-energy in matter: collisions and annihilation on baryons[1] (imaginary part, semiclassically given by 2ImV = σvρ) and a piece in the real part (ReV = t NN ρ in the impuls approximation). In the semiclassical limit the real part of the self-energy can be approximated by potential-type interaction[2] or a mean field. Here we will focus on the effect of the real part. The motivation is that the long-range force of baryons acting on ap is expected to be stronger than for protons since the Lorentz-scalar and the Lorentz-vector parts of a meson exchange potential now have the same sign. The influence of baryonic mean-fields on baryons and mesons is well established. Therefore there should also be some influence onp's. The substantial impact of mean-fields on particle spectra was studied earlier [3,4]. Following these ideas, we now investigate observables in nucleus-nucleus collisions, where BB-potentials come into play. For this purpose, we employed a simple modelinteraction, knowing that this choice is far from being unique. In principle, one has to calculate the medium-dependent mean-field and the crosssection selfconsistently to understand N behaviour in matter. We calculate the forces acting on a N in a baryonic medium only after freeze-out. However, by taking the free interaction -annihilation, elastic scattering, non-annihilating inelastic channels -for NN in the collision term during the dynamical evolution the real potential is included effectively. Our approach is similar in its spirit to the usual treatment of the Coulomb distortion of particle spectra which is also restricted to final-state interactions [5]. Addition of free interactions and mean-field contribution would cause 1

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We present a RQMD calculation of antiproton yields and their momentum distribution in Ne+NaF collisions at 2 GeV /u. The antiprotons can be produced below threshold due to multi-step excitations for which meson-baryon interactions play a considerable role. In this system the annihilation probability for an initially produced antiproton is predicted to be about 65%.