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We calculate the momentum dependence of the ρ-meson selfenergy based on the dispersion relation for the ρN scattering amplitude f (ω) at low nuclear density. The imaginary part of f (ω) is determined from the optical theorem, while the total ρN cross section is obtained within the VDM at high energy and within the resonance model at low energy. Our numerical results indicate a sizeable broadening of the ρ-meson width in the medium especially for low relative momenta p while the real part of the ρ selfenergy is found to change its sign and to become repulsive already at momenta above 100 MeV/c. Extrapolating to nuclear saturation density ρ 0 we find a dropping of the ρ-mass for p ≈ 0 roughly in line with the QCD sumrule analysis of Hatsuda while at high energy an increase of the ρ-mass close to the prediction by Eletsky and Joffe is obtained. However, when including a broadening of the baryonic resonances in the medium, the ρmeson mass shift at p ≈ 0 becomes slightly repulsive whereas the width increases substantially.

The distributions of residual nuclei after annihilation of stopped antiprotons in 92M0, 95M0, 98M0 and 165H0 targets have been measured by means of the induced radioactivity. In the case of the 165H0 target the residual nucleus ~ 16Te was observed thus indicating that about 50 nucleons may be emitted after annihilation. The distributions have also been calculated with two versions of an intranuclear cascade model. The agreement between theory and experiment is satisfactory. The effects of heavy mesons in the annihilation process, of local reduction of the nuclear density during the intranuclear cascade and of multifragmentation are discussed.

The annihilation of energetic (1.2 GeV) antiprotons is exploited to deposit maximum thermal excitation (up to 1000 MeV) in massive nuclei (Cu, Ho, Au, and U) while minimizing the contribution from collective excitation such as rotation, shape distortion, and compression. Excitation energy distributions ds͞dE ‫ء‬ are deduced from eventwise observation of the whole nuclear evaporation chain with two 4p detectors for neutrons and charged particles. The nuclei produced in this way are found to decay predominantly statistically, i.e., by evaporation.[ S0031-9007(96) The study of such decay modes of very highly excited nuclei as fission, multifragmentation, cracking, and vaporization is presently a major objective in nuclear physics because of its bearing on the lesser-known bulk properties of hot nuclear matter, such as heat capacity, specific heat, viscosity, and phase transitions. Unfortunately, the decay pattern is also very sensitive to the dynamics of the excitation process, especially when collective degrees of freedom like rotation, shape distortion, and compression are strongly induced. These may have to be envisaged in the most often used [1-3] heavy-ion reactions. This ambiguity makes it difficult to correlate the observed decay pattern with either thermally or dynamically induced decay.In order to minimize the influence of the entrance channel on the decay modes, we have, for the first time, investigated the nuclear excitation following annihilation of energetic antiprotons. Antiprotons annihilate on a single nucleon at the surface of, or even inside the nucleus, thereby producing a pion cloud containing an average of about 5 particles. Because of the high centerof-mass velocity (b c.m.0.63) of this cloud, it is focused forward into the nucleus. Since the pion momenta are comparable to the Fermi momentum of the nucleons in the nucleus, the pions heat the nucleus in a soft radiationlike way [4], probably even softer and more efficient than can be expected in proton-or other lightion-induced spallation reactions, which have also been exploited recently for this purpose [5][6][7].Intranuclear cascade (INC) calculations have been found to provide a reasonable description of this mechanism. They predict that the spin remains low (below maximum 25h) and that shape distortion and density compression are negligible [8], in contrast to what is expected in heavy-ion reactions. The reaction time for achievement of equilibrium conditions is only about 30 fm͞c or 10 222 s [9], which is much shorter in general than the dynamical period in heavy-ion reactions [10]. This is all the more important at high temperature (T ഠ 6 MeV) when the characteristic evaporation time reduces to t , 10 222 s, implying little cooling of the compound nucleus during heating.In this Letter we concentrate on the use of a new method to determine the thermal excitation energy produced with energetic antiprotons. This method is based on the eventwise observation of the whole nuclear evaporation chain, including both neutrons and charged particles...

The ω-and ρ-resonance production and their dileptonic decay in π − A reactions at GSI energies are calculated within the intranuclear cascade (INC) approach. The invariant mass distribution of the dilepton pair for each resonance is found to have two components which correspond to the decay of the resonances outside and inside the target nucleus. The latter components are strongly distorted by the nuclear medium due to resonance-nucleon scattering and a possible mass shift at finite baryon density. These medium modifications are compared to background sources in the dilepton spectrum from πN bremsstrahlung and the Dalitz decays of ∆'s, ω and η mesons produced in the reaction. * Supported by DFG 1The question about the properties of hadronic resonances in the nuclear medium has received a vivid attention during the last years (cf. Refs. [1,2,3,4]). Here, QCD inspired effective Lagrangian models [1,2] or approaches based on QCD sum rules [3,4] predict that the masses of the vector mesons ρ, ω and φ should decrease with the nuclear density. Furthermore, along with a dropping mass the phase space for the resonance decay also decreases which results in a modification of the resonance width in matter. On the other hand, due to collisional broadening -which depends on the nuclear density and the resonance-nucleon interaction cross section ( cf. Refs. [5,6]) -the resonance width should increase again.The in-medium properties of vector mesons have been addressed experimentally so far by dilepton measurements at the SPS, both for proton-nucleus and nucleusnucleus collisions [7,8,9]. As proposed by Li et al. [10], the enhancement in S + Au reactions compared to p + Au collisions in the invariant mass range 0.3 ≤ M ≤ 0.7GeV might be due to a shift of the ρ meson mass. The microscopic transport studies in Refs. [11,12] for these systems point in the same direction, however, also more conventional selfenergy effects cannot be ruled out at the present stage [11,13]. It is therefore necessary to have independent information on the vector meson proporties from reactions, where the dynamical picture is more transparent, i.e. in pion-nucleus collisions. Here, especially the ω meson can be produced with low momenta in the laboratory system, such that a substantial fraction of them will still decay inside a heavy nucleus [14].The mass distributions of the vector mesons in the latter case are expected to have a two component structure [6] in the dilepton invariant mass spectrum: the first component corresponds to resonances decaying in the vacuum, thus showing the free spectral function which is very narrow in case of the ω meson; the second (broad) component then corresponds to the resonance decay inside the nucleus. We will use that (in first order) the in-medium resonance can also be described by a Breit-Wigner formula with a mass and width distorted by the nuclear environment.In this letter we present first microscopic calculations for the production and dilep- The vector mesons ρ, ω are produced in the first hard pion-nu...