Collective flow measurements with HADES in Au+Au collisions at 1.23A GeV

Abstract

HADES has a large acceptance combined with a good mass-resolution and therefore allows the study of dielectron and hadron production in heavy-ion collisions with unprecedented precision. With the statistics of seven billion Au-Au collisions at 1.23A GeV recorded in 2012, the investigation of higher-order flow harmonics is possible. At the BEVALAC and SIS18 directed and elliptic flow has been measured for pions, charged kaons, protons, neutrons and fragments, but higher-order harmonics have not yet been studied. They provide additional important information on the properties of the dense hadronic medium produced in heavy-ion collisions. We present here a high-statistics, multi-differential measurement of v 1 and v 2 for protons in Au+Au collisions at 1.23A GeV.

First, second, third and fourth flow harmonics of deuterons and protons in Au+Au reactions at 1.23 AGeV

“…Figure 1 shows the directed flow of free protons as a function of rapidity for various transverse momentum regions. The lines denote the model calculations and the symbols show the preliminary HADES data [39,40]. In comparison to the results shown in [28], we observe an improved description of the low transverse momentum proton data if deuteron formation is taken into account.…”

“…Figures 5 and 6 show the elliptic flow v 2 of protons and deuterons as a function of rapidity for various transverse momentum regions at a fixed target beam energy of 1.23 AGeV. The lines denote the model calculations and the data points the preliminary HADES data [39,40,42]. For both protons and deuterons one observes a decrease of v 2 with increasing p T indicating a stronger absorption of high transverse momentum protons/deuterons, due to their earlier emission.…”

“…UrQMD, free p HADES AGeV. The symbols denote the experimental data (20%-30% centrality) [39,40], the lines indicate the UrQMD calculations (b = 6 − 9 fm).…”

“…Figures 7 and 8 show the elliptic flow v 2 as a function of transverse momentum for different rapidity bins for Au+Au collisions at a beam energy of 1.23 AGeV. The lines denote the UrQMD calculations (b = 6 − 9 fm) and the symbols denote the preliminary experimental data (20%-30% centrality) [39,40,42].…”

“…the triangular flow, have only been studied as a consequence of initial state fluctuations at RHIC and LHC energies. The HADES experiment at the SIS18 accelerator at GSI in Darmstadt has measured Au+Au collisions at a fixed target beam energy of 1.23 AGeV, collecting a significant amount of data, sufficient to determine not only v 1 and v 2 , but also higher flow components with the charged projectile spectators event plane method [38][39][40]42] which we approximate with the theoretical reaction plane. While initial state fluctuations (that drive the odd flow components at higher energies) are not connected to the participant reaction plane [33][34][35][36][37] one usually expected that triangular flow can not be observed with a fixed participant plane.…”