The impact of energy conservation in transport models on the π−/π+ multiplicity ratio in heavy-ion collisions and the symmetry energy

M.D Cozma

doi:10.1016/j.physletb.2015.12.015

Abstract

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.

Highlights

The isovector part of the equation of state of nuclear matter, commonly known as symmetry energy (SE), has an important impact on the structure of rare isotopes, dynamics and spectra of heavy-ion collisions and on certain astrophysical processes such as neutron star cooling, their chemical composition, and supernovae explosions [1, 2]
It was found that the choice K=245 MeV coupled with medium modifications of both elastic and inelastic channels cross-sections allows a good reproduction of experimental pion multiplicities together with a fair onefor elliptic flow values
An upgraded version of the Tubingen QMD transport model, which allows the conservation of energy in intermediate energy heavy-ion collisions at an event by event basis, has been presented

Summary

Introduction

The isovector part of the equation of state of nuclear matter (asy-EoS), commonly known as symmetry energy (SE), has an important impact on the structure of rare isotopes, dynamics and spectra of heavy-ion collisions and on certain astrophysical processes such as neutron star cooling, their chemical composition, and supernovae explosions [1, 2]. It is of uttermost importance to study experimentally nuclear matter at suprasaturation densities, a regime reached in Earth based laboratories only during the process of heavy-ion collisions (HICs) To this end several promising observables have been identified: the ratio of neutron/proton yields of squeezed out nucleons [5], light cluster emission [6], π−/π+ multiplicity ratio (PMR) in central collisions [7, 8, 9], elliptic flow related observables [10] and others. Constraints for the asy-EoS stiffness from the elliptic flow ratio of neutrons vs hydrogen and neutrons vs protons have been extracted using the UrQMD transport model and power-law [11] and contact Skyrme interactions [12] parametrizations of the symmetry potential with the results for the slope parameter of SE at saturation L=83±52 MeV and L=89±45 MeV respectively. In all cases reanalyzed sets of the 90’s experimental data for 197Au+197Au collisions at an incident projectile energy of 400 MeV/nucleon, obtained by the FOPI-LAND collaboration [15, 16], have been employed

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Conclusion