Probing system size dependence at high baryon density by systematic comparison of Ag+Ag and Au+Au reactions at 1.23$A$ GeV

TL;DR

This study compares Ag+Ag and Au+Au collisions at 1.23A GeV using UrQMD predictions, revealing system size dependence and flow scaling related to initial state eccentricity, providing insights into high baryon density matter.

Contribution

It introduces a systematic comparison method for different collision systems at low energies, emphasizing the importance of centrality criteria and initial state eccentricity scaling.

Findings

Good agreement between systems when using appropriate centrality measures.

Particle yields scale with the number of participants, except in very central reactions.

Elliptic flow follows initial eccentricity scaling with opposite sign to high energies.

Abstract

We present UrQMD predictions for the comparison of the recently measured Ag+Ag and Au+Au runs at a beam energy of $E_\mathrm{lab}$ = 1.23~$A$GeV explored by the HADES experiment. To this aim, different centrality definitions are investigated: We suggest that in the case of particle production, both systems should be compared at the same number of participating nucleons, while for a comparison of the (elliptic) flow, a selection on the initial state eccentricity - as in high energy reaction - is better suited. Generally, we find good agreement between both system, if these centrality criteria are used. Specifically, the deuteron yields per participant and the pion to proton ratios are shown to scale with $A_\mathrm{part}$ except for very central Ag+Ag reactions due to stronger stopping in such reactions. The elliptic flow in both systems follows inital state eccentricity scaling, albeit with the opposite sign as compared to high energies, suggesting a strong relation between final flows and the initial state also at the low energies explored here. The observation of this scaling might then allow to obtain further information on the expansion properties (and the EoS) of matter at high baryon densities.