Among the light nuclear clusters the α-particle is by far the strongest bound system and therefore expected to play a significant role in the dynamics of nuclei and the phases of nuclear matter. To systematically study the properties of the α-particle we have derived an effective four-body equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant medium effects, i.e. self energy corrections and Pauli-blocking in a consistent way. The equation is solved utilizing the energy dependent pole expansion for the subsystem amplitudes. We find that the Mott transition of an α-particle at rest differs from that expected from perturbation theory and occurs at approximately 1/10 of nuclear matter densities.

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Within QMD simulations we demonstrate the effect of virial corrections on heavy ion reactions. Unlike in standard codes, the binary collisions are treated as non-local so that the contribution of the collision flux to the reaction dynamics is covered. A comparison with standard QMD simulations shows that the virial corrections lead to a broader proton distribution bringing theoretical spectra closer towards experimental values. Complementary BUU simulations reveal that the non-locality enhances the collision rate in the early stage of the reaction. It suggests that the broader distribution appears due to an enhanced pre-equilibrium emission of particles.The Boltzmann equation including the Pauli blocking (the BUU equation [1]) and the closely related method of quantum molecular dynamics [2,3] (QMD) are extensively used to interpret experimental data from heavy ion reactions. Due to their quasi-classical character, they offer a transparent picture of the internal dynamics of reactions and allow one to link observed particle spectra with individual stages of reactions.An ambition to cover the heavy ion reactions within experimental errors has been recently cooled down by a failure of BUU simulations to describe the energy and angular distribution of neutrons and protons in low and mid energy domain [4][5][6]. Indeed, the Boltzmann equation is not the full story. As noticed in numerical studies of hard sphere cascade by Halbert [7] and more general by Malfliet [8], it is disturbing that all dynamical models rely more or less on the use of the space-and timelocal approximation of binary collisions inherited from the Boltzmann equation. This approximation neglects a contribution of the collision flux to the compressibility and the share viscosity which control the hydrodynamic motion during the reaction. To include the collision flux and other virial corrections, the non-local character of binary collisions has to be accounted for. Malfliet also demonstrated that non-local collisions can be easily incorporated into simulation BUU codes.In absence of a first principle theory, Malfliet in his pioneering study, and more recently Kortemeyer, Daffin and Bauer [9], had to use classical hard-sphere-like non-local collisions which naturally do not result in a full quantitative agreement with experimental data. This ad hoc approximation reflects a gap in former quasi-classical theories of quantum transport: authors either cared about non-local collisions leaving aside quasiparticle features or vice versa. Moreover, quantum theories of binary collisions treated non-local collisions via gradient contributions to the scattering integral [10,11] which are numerically inconvenient and thus have never been employed in demanding simulations of heavy ion reactions. Recent theoretical studies have filled this gap. Danielewicz and Pratt [12] pointed out that the collision delay can be used as a convenient tool to describe the virial corrections to the equation of state for the gas of quasiparticles. Although their discussion is limit...

Orthonormal spin-flavor wave functions of Lorentz covariant quark models of the Bakamjian-Thomas type are constructed for nucleon resonances. Three different bases are presented. The manifestly Lorentz covariant Dirac-Melosh basis is related to the Pauli-Melosh basis and the symmetrized BargmannWigner basis that are manifestly orthogonal.

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Within a microscopic statistical description of heavy ion collisions, we investigate the effect of the medium on the formation of light clusters. The dominant medium effects are self-energy corrections and Pauli blocking that produce the Mott effect for composite particles and enhanced reaction rates in the collision integrals. Microscopic description of composites in the medium follows the Dyson equation approach combined with the cluster mean-field expansion. The resulting effective few-body problem is solved within a properly modified Alt-Grassberger-Sandhas formalism. The results are incorporated in a Boltzmann-Uehling-Uhlenbeck simulation for heavy ion collisions. The number and spectra of light charged particles emerging from a heavy ion collision changes in a significant manner in effect of the medium modification of production and absorption processes.

We investigate deuteron formation in nuclear matter at finite temperatures within a systematic quantum statistical approach. We consider formation through three-body collisions relevant already at rather moderate densities because of the strong correlations. The three-body in-medium reaction rates driven by the break-up cross section are calculated using exact three-body equations (Alt-Grassberger-Sandhas type) that have been suitably modified to consistently include the energy shift and the Pauli blocking. Important quantities are the lifetime of deuteron fluctuations and the chemical relaxation time. We find that the respective times differ substantially while using in-medium or isolated cross sections. We expect implications for the description of heavy ion collisions in particular for the formation of light charged particles at low to intermediate energies.Comment: 19 pages, 5 figure

We consider deuteron formation in heavy ion collisions at intermediate energies. The elementary reaction rates (N d → N N N etc.) in this context are calculated using rigorous Faddeev methods. To this end an in-medium Faddeev equation that consistently includes the energy shift and Pauli blocking effects has been derived and solved numerically. As a first application we have calculated the life-time of deuteron fluctuations for nuclear densities and temperatures typical for the final stage of heavy ion collisions. We find substantial differences between using the isolated and the in-medium rates. * Work supported by the Deutsche Forschungsgemeinschaft. † Talk presented by M. Beyer.