Collisions of Au on Au at incident energies of 150, 250 and 400 A MeV were studied with the FOPI-facility at GSI Darmstadt. Nuclear charge (Z ≤ 15) and velocity of the products were detected with full azimuthal acceptance at laboratory angles 1 • ≤ θ lab ≤ 30 • . Isotope separated light charged particles were measured with movable multiple telescopes in an angular range of 6 − 90 • . Central collisions representing about 1% of the reaction cross section were selected by requiring high total transverse energy, but vanishing sideflow. The velocity space distributions and yields of the emitted fragments are reported. The data are analysed in terms of a thermal model including radial flow. A comparison with predictions of the Quantum Molecular Model is presented.PACS: 25.70.Pq

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The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the $p_t$ spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of $(p, \pi^{\pm})$ pairs. With both methods the deduced mass distributions are shifted by an average value of -60 MeV/c$^2$ relative to the mass distribution of the free $\Delta(1232)$ resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c^2. The observed differences between $(p, \pi^-)$ and $(p, \pi^+)$ pairs indicate a contribution of isospin $I = 1/2$ resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities

Four different combinations of 9644 Ru and 96 40 Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400A MeV using a 4π detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the t/ 3 He yield ratio. The experimental results show that the global equilibrium is not reached even in the most central collisions. Quantitative measures of stopping and mixing are extracted from the data. They are found to exhibit a quite strong sensitivity to the in-medium (n,n) cross section used in microscopic transport calculations.PACS numbers: 25.75.Dw; 25.75.Ld Central heavy-ion collisions represent a valuable tool for studies of hot and dense nuclear matter where one hopes to infer valuable information on the nuclear equation of state (EOS) and on modifications of hadrons in the nuclear medium. It is still an open question whether the widely applied, at least local if not global, equilibrium assumption is valid in such reactions [1,2], or whether significant non-equilibrium effects rather require the application of more elaborated non-equilibrium dynamical models [2][3][4]. The issue of equilibration is expected to be influenced by in-medium effects (such as Pauli blocking, Fermi motion) on the 'hard' scattering processes, by early 'soft' deflections in the momentum-dependent mean fields, and by finite-size (corona) effects. An understanding of all these effects is a prerequisite for a quantitative extraction of the EOS from nucleus-nucleus collisions.Experimental observations of non-equilibrium in relativistic heavy-ion collisions were concentrated, up to now, on the measurement of the momentum distribution of the products emerging from a mid-rapidity "source" of symmetric colliding systems. Observables of interest were the width of rapidity distributions [3,5] or the overall shape of the source [6,7]. The sensitivity of such observables is however reduced by effects like rescattering during the late phase of expansion.In order to extract, in a model independent approach, direct experimental information on non-equilibrium we have designed a new type of high precision measurement which makes use of the isospin (N/Z) degree of freedom. The (N/Z) equilibration has been investigated before at low bombarding energies in fusion-like reactions [8][9][10]. As it will be shown in some details later, (N/Z) can be used as a tracer in order to attribute the measured nucleons (on average) either to the target or to the projectile nucleons. It is therefore possible to extract rapiditydensity distributions separately for projectile and target nucleons. This gives access to new more sensitive observables, like stopping and mixing, of the early equilibration process.The experiment was carried-out using reactions between equal mass nuclei A = 96, at an incident energy of 400A MeV. Isotopes of Ru and Zr were taken as projectile and target making ...

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