Neutron-proton elliptic flow difference as a probe for the high density dependence of the symmetry energy

TL;DR

This study uses an isospin-dependent transport model to analyze how neutron-proton elliptic flow differences can reveal the high-density behavior of the nuclear symmetry energy, despite model sensitivities.

Contribution

It demonstrates that neutron-proton elliptic flow difference is a robust observable for probing the high-density symmetry energy, less affected by model uncertainties.

Findings

Neutron-proton elliptic flow difference is less sensitive to model parameters.

Experimental data exclude extreme super-soft symmetry energy scenarios.

Current data cannot further constrain the symmetry energy due to uncertainties.

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. Comparison with experimental neutron-hydrogen FOPI-LAND data excludes the extreme super-soft scenarios for the high density dependence of asy-EoS. Existing neutron-proton elliptic flow difference experimental data are poised with uncertainties and cannot constrain the asy-EoS any further.