Hydrodynamics at large baryon densities: Understanding proton versus anti-protonv2and other puzzles

Abstract

We study the importance of the initial state, baryon stopping and baryon number transport for the dynamical evolution of a strongly interacting system produced in heavy ion collisions. We employ a hybrid model, which combines the fluid dynamical evolution of the fireball with a transport treatment for the initial state and the final hadronic phase. We present results for collisions at beam energies from √ sNN = 7.7 to 200 GeV. We study various observables such as the centrality dependent freeze out parameters, the non-monotonic behavior of effective slope parameter parameter with particle mass as well as the apparent difference in particle and anti-particle elliptic flow. Our results are in reasonable agreement with the available data. We find that the propagation of the baryon-number current in the hydrodynamic evolution as well as the transport treatment of the hadronic phase are essential for reproducing the experimental data.

An explanation of the elliptic flow difference between proton and anti-proton from the UrQMD model with hadron potentials

“…This is similar to calculation results with UrQMD/H in Ref. [5]. From 8 to 12 fm/c, the central density falls to the subnormal densities, which has caused a bulky attractive-potential effect.…”

“…Besides confirmations on previous measurements by collaborations at CERN-SPS, more attention has been drawn on new findings, such as the elliptic flow splitting of particles especially of protons and anti-protons seen at low BES energies [4]. While a constituent quark number scaling of the hadronic elliptic flow indicates the existence of the QGP phase, the splitting might hint the change of its dynamical evolution or even its disappearance which has been explained by several theoretical groups [5][6][7][8][9].…”

“…[5], with the help of the so-called Ultra-relativistic Quantum Molecular Dynamics (UrQMD) hybrid model in which the intermediate hot and dense phase is modeled with an ideal hydrodynamic evolution (called UrQMD/H), Steinheimer et al found that the difference in elliptic flow of protons and anti-protons is pronounced just after the hydrodynamical phase (due to the difference in chemical potential) while it is almost washed out by the subsequent transport phase in the cascade mode (due to the dominant annihilation process). In Ref.…”

Elliptic Flow Splitting as a Probe of the QCD Phase Structure at Finite Baryon Chemical Potential

“…This can then lead to the splitting of the v 2 of oppositely charged particles and antiparticles, particularly that of π + and π − due to their similar final-state interactions in the hadronic matter. The v 2 splitting of particles and antiparticles may also be attributed to different v 2 of transported and produced partons [12], different rapidity distributions of quarks and antiquarks [13], and the conservation of baryon charge, strangeness, and isospin [14].…”

“…This can then lead to the splitting of the v 2 of oppositely charged particles and antiparticles, particularly that of π + and π − due to their similar final-state interactions in the hadronic matter. The v 2 splitting of particles and antiparticles may also be attributed to different v 2 of transported and produced partons [12], different rapidity distributions of quarks and antiquarks [13], and the conservation of baryon charge, strangeness, and isospin [14].On the other hand, we have shown in our previous studies that the different mean-field potentials for hadrons and antihadrons [15] or quarks and antiquarks [16] in the baryon-rich matter produced at lower collision energies can describe qualitatively the v 2 splitting of particles and their antiparticles. This is due to the fact that particles with attractive potentials are more likely to be trapped in the system and move in the direction perpendicular to the participant plane, while those with repulsive potentials are more likely to leave the system and move along the participant plane, thus reducing and enhancing their respective elliptic flows.…”