W. Cassing

Production of energetic particles in heavy-ion collisions☆

Abstract The investigation of hot and compressed nuclear matter by means of heavy-ion collisions requires the development of probes that are sensitive to the time evolution of the nuclear phase-space density. Hard particles, such as energetic photons or mesons, which are produced during the reaction, may provide such probes. In the first part of this article we describe the time evolution of the nuclear many-body system within a nonperturbative transport approach as well as the methods employed for the evaluation of the elementary nucleon-nucleon-particle cross sections. The second part contains a description of the experimental set-ups used for the detection of hard photons in heavy-ion experiments and a discussion of the empirical systematics of data gained so far. In the third part of this article we compare the theoretical predictions for differential particle yields with the available data. Detailed comparisons are presented for hard photons at bombarding energies from 15 MeV/u to 125 MeV/u, for dileptons at 1 GeV/u and for mesons (πs, ηs, K+s) from 30MeV/u to 2 GeV/u. We finally discuss selection criteria for optimal probes with respect to the experimental determination of the nuclear equation of state at high densities.

Dilepton production in heavy-ion collisions

Abstract The dynamical evolution of nucleus-nucleus collisions is described by a transport equation of the Uehling-Uhlenbeck type. Our model evolves phase-space distribution functions for nucleons, Δs, N ∗ (1440)-resonances and pions with their isospin degrees of freedom. The equations are solved by the test-particle simulation method. We apply this model to proton-nucleus and nucleus-nucleus collisions from 400 MeV/ A to 2.1 GeV/ A . Our approach is found to correctly describe the spectra of baryons and pions. Furthermore, we calculate the production of dileptons considering proton-neutron and pion bremsstrahlung, Δ and N ∗ decay and π + π − annihilation. The calculations reproduce the data for p+ 9 Be and for 40 Ca + 40 Ca at the bombarding energies of 1 GeV/ A and 2 GeV/ A except for small invariant masses ( M ⩽ 200 MeV) where the situation is not fully understood. Above 300 MeV dilepton invariant mass the relevant contributions arise from proton-neutron bremsstrahlung, the Δ Dalitz-decay and π + π − annihilation. At 2 GeV/ A for 40 Ca + 40 Ca, however, we find the bremsstrahlung contribution to be higher than that from pion annihilation.

Relativistic transport approach for nucleus-nucleus collisions from SIS to SPS energies☆☆☆

Abstract We formulate a covariant transport approach for high-energy nucleus-nucleus collisions where the real part of the nucleon self-energies is fitted to nuclear-matter properties which are evaluated on the basis of a NJL-type Lagrangian for the quark degrees of freedom. The parameters of the quark-model Lagrangian are fixed by the Gell-Mann, Oakes and Renner relation, the pion-nucleon Σ-term, the nucleon energy as well as the nuclear binding energy at saturation density ϱ 0 . We find the resulting scalar and vector self-energies for nucleons to be well in line with either Dirac-Brueckner computations for ϱ ⩽ 2 ϱ 0 or those from the phenomenological optical potential when accounting for a swelling of the nucleon at finite nuclear-matter density. The meson-baryon interaction density is modeled to describe a decrease of the meson mass with baryon density. The imaginary part of the hadron self-energies is determined by a string fragmentation model which accounts for the in-medium mass of hadrons in line with the ‘chiral’ dynamics employed. The applicability of the transport approach is demonstrated in comparison with experimental data from SIS to SPS energie The enhancement of the K + π + ratio in A + A collisions compared to p + A reactions at AGS energies is reproduced within the ‘chiral’ dynamics. Furthermore, detailed predictions for the stopping in Pb + Pb collisions at 158 GeV A are presented.

Dilepton production at SPS energies

Abstract We present a dynamical study of e+e− production in proton-nucleus and nucleus-nucleus collisions at SPS energies on the basis of a covariant transport approach. For p + A reactions the dilepton yield for invariant masses m ≤ 1.2 GeV is found to be dominated by the decays of the η, ϱ, ω and Φ mesons in line with the findings of the CERES Collaboration. For S + Au collisions at 200 GeV/A the dilepton yield is, however, dominated by π+π− annihilation due to the high pion densities achieved. For ‘free’ meson masses and form factors the experimental cross section is slightly underestimated for 0.3 GeV ≤ m ≤ 0.45 GeV. Effects due to different medium modification of the ϱ-meson are studied in comparison to the CERES data.

Kaon versus antikaon production at SIS energies

Abstract We analyse the production and propagation of kaons and antikaons in Ni + Ni reactions from 0.8–1.85 GeV/u within a coupled channel transport approach including the channels BB → K+YN, πB → K+Y, BB → NNK K , πB → NK K , K+B → K+B, K B → K B , YN → K NN , ππ → K K as well as πY → K N and K N → πY for the antikaon absorption. Whereas the experimental K+ spectra can be reproduced without introducing any self-energies for the mesons in Ni + Ni collisions from 0.8 to 1.8 GeV/u, the K− yield is underestimated by a factor of 5–7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon masses in line with effective chiral Lagrangians, the antikaon spectra can be reasonably well described.

Strangeness dynamics and transverse pressure in relativistic nucleus-nucleus collisions

We investigate hadron production as well as transverse hadron spectra from proton-proton, proton-nucleus and nucleus-nucleus collisions from 2

Semiclassical transport of particles with dynamical spectral functions

A\cdot

Analysis of kaon production at SIS energies

GeV to 21.3

A relativistic effective interaction for heavy-ion collisions☆☆☆

A\cdot

Bethe-Salpeter Meson Masses Beyond Ladder Approximation

TeV within two independent transport approaches (HSD and UrQMD) that are based on quark, diquark, string and hadronic degrees of freedom. The comparison to experimental data on transverse mass spectra from