Directed, elliptic and triangular flow of protons in Au+Au reactions at 1.23 AGeV: A theoretical analysis of the recent HADES data

TL;DR

This paper uses the UrQMD transport model to analyze flow harmonics of protons in Au+Au collisions at 1.23 AGeV, highlighting the sensitivity of triangular flow to the nuclear equation of state and providing new insights into low-energy nuclear dynamics.

Contribution

First theoretical calculation of triangular flow and its excitation function at low energies, demonstrating the role of potential interactions in flow development.

Findings

Flow harmonics are consistent with potential interactions in the model.

Triangular flow is sensitive to the nuclear equation of state.

Softening of the equation of state above 7 A GeV is indicated.

Abstract

Recently, the HADES experiment at GSI has provided preliminary data on the directed flow, $v_1$ elliptic flow, $v_2$ and triangular flow, $v_3$ of protons in Au+Au reactions at a beam energy of 1.23 A GeV. Here we present a theoretical discussion of these flow harmonics within the UrQMD transport approach. We show that all flow harmonics, including the triangular flow, provide a consistent picture of the expansion of the system, if potential interactions are taken into account. Investigating the dependence of the flow harmonics on the nuclear interaction potentials it is shown that especially $v_3$ can serve as a sensitive probe for the nuclear equation of state at such low energies. The triangular flow and its excitation function with respect to the reaction-plane were calculated for the first time and indicate a complex interplay of the time-evolution of the system and the initial conditions at low beam-energies. Our study also indicates a significant softening of the equation of state at beam energies above $E_{\mathrm{lab}}> 7$A GeV which can be explored by at the future FAIR facility.