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The charged pion multiplicity ratio in intermediate energy central heavy-ion collisions has been proposed as a suitable observable to constrain the high density dependence of the isovector part of the equation of state. A comparison of various transport model predictions with existing experimental data has led, however, to contradictory results. Using an upgraded version of the Tübingen QMD transport model, which allows the conservation of energy at a local or global level by accounting for the potential energy of hadrons in two-body collisions and leading thus to particle production threshold shifts, we demonstrate that compatible constraints for the symmetry energy stiffness can be extracted from pion multiplicity and elliptic flow observables. However, pion multiplicities and ratios are proven to be highly sensitive to the yet unknown isovector part of the in-medium Δ ( 1232 ) potential which hinders, at present, the extraction of meaningful information on the high density dependence of the symmetry energy. A solution to this problem together with the inclusion of contributions presently neglected, such as in-medium pion potentials and retardation effects, are needed for a final verdict on this topic. [ABSTRACT FROM AUTHOR]

Abstract: We employ an isospin dependent version of the QMD transport model to study the influence of the isospin dependent part of the nuclear matter equation of state and in-medium nucleon–nucleon cross-sections on the dynamics of heavy-ion collisions at intermediate energies. We find that the extraction of useful information on the isospin-dependent part of the equation of state of nuclear matter from proton or neutron elliptic flows is obstructed by their sensitivity to model parameters and in-medium values of nucleon–nucleon cross-sections. Opposite to that, neutron–proton elliptic flow difference shows little dependence on those variables while its dependence on the isospin asymmetric EoS is enhanced, making it more suitable for a model independent constraining of the high-density behaviour of asy-EoS. Comparison with existing experimental FOPI-LAND neutron–hydrogen data can be used to set an upper limit to the softness of asy-EoS. Successful constraining of the asy-EoS via neutron–proton elliptic flow difference will require experimental data of higher accuracy than presently available. [Copyright &y& Elsevier]

Abstract: We employ a QMD transport model to study the influence of the isospin dependent part of the nuclear matter equation of state on the dynamics of heavy-ion collisions at intermediate energies. Parametrization of the in-medium dependence of the nucleon–nucleon elastic cross-sections, as predicted by microscopical models, are used. The sensitivity of flow observables to various parametrizations of the isospin dependent part of the equation of state (symmetry energy) is studied. The experimentally observed splitting of the elliptic flow values in Ru+Zr and Zr+Ru at incident energies of 400 MeV is shown to be due to, to an equal extent, density dependence of the microscopic nucleon–nucleon cross-section and symmetry energy. [Copyright &y& Elsevier]