Hadron-hadron collisions at high energies are investigated in the Ultrarelativistic-Quantum-Molecular-Dynamics approach. This microscopic transport model describes the phenomenology of hadronic interactions at low and intermediate energies ( √ s < 5 GeV) in terms of interactions between known hadrons and their resonances. At higher energies, √ s > 5 GeV, the excitation of color strings and their subsequent fragmentation into hadrons dominates the multiple production of particles in the UrQMD model. The model shows a fair overall agreement with a large body of experimental h-h data over a wide range of h-h center-of-mass energies. Hadronic reaction data with higher precision would be useful to support the use of the UrQMD model for relativistic heavy ion collisions.

No abstract

Dielectron mass spectra are examined for various nuclear reactions recently measured by the DLS Collaboration. A detailed description is given of all dilepton channels included in the transport model UrQMD 1.0, i.e., Dalitz decays of 0 ,,,Ј mesons and of the ⌬(1232) resonance, direct decays of vector mesons and pn bremsstrahlung. The microscopic calculations reproduce data for light systems fairly well, but tend to underestimate the data in pp at high energies and in pd at low energies. These conventional sources, however, cannot explain the recently reported enhancement for nucleus-nucleus collisions in the mass region 0.15 GeVрM e ϩ e Ϫр 0.6 GeV. Chiral scaling and meson broadening in the medium are investigated as a source of this mass excess. They also cannot explain the recent DLS data. ͓S0556-2813͑98͒01007-3͔PACS number͑s͒: 25.75.Ϫq † Present address:

The mass shift, width broadening, and spectral density for and mesons in a heat bath of nucleons and pions are calculated using a general formula which relates the self-energy to the real and imaginary parts of the forward scattering amplitude. We use experimental data to saturate the scattering amplitude at low energies with resonances and include a background Pomeron term, while at high energies a Regge parametrization is used. The real part obtained directly is compared with the result of a dispersion integral over the imaginary part. The peaks of the spectral densities are little shifted from their vacuum positions, but the widths are considerably increased due to collisional broadening. Where possible we compare with the UrQMD model and find quite good agreement. At normal nuclear matter density and a temperature of 150 MeV the spectral density of the meson has a width of 345 MeV, while that for the is in the range 90-150 MeV. 64 035202-1 FIG. 6. The vector meson widths as a function of momentum p. Results are shown for nucleon densities of 0, n 0 , and 2n 0 ͑where equilibrium nuclear matter density n 0 ϭ0.16 fm Ϫ3 ) and temperatures of 100 and 150 MeV. For the meson results are given for the multiresonance and the two-resonance models.PROPERTIES OF and MESONS AT FINITE . . . PHYSICAL REVIEW C 64 035202 035202-7