Anisotropic emission of thermal dielectrons from Au+Au collisions at $\sqrt{s_{NN}}=200$~GeV with EPOS3

Abstract

Dileptons, as an electromagnetic probe, are crucial to study the properties of a Quark-Gluon Plasma (QGP) created in heavy ion collisions. We calculated the invariant mass spectra and the anisotropic emission of thermal dielectrons from Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) energy √ sNN = 200 GeV based on EPOS3. This approach provides a realistic (3+1)-dimensional event-by-event viscous hydrodynamic description of the expanding hot and dense matter with a very particular initial condition, and a large set of hadron data and direct photons (besides v2 and v3 !) can be successfully reproduced. Thermal dilepton emission from both the QGP phase and the hadronic gas are considered, with the emission rates based on Lattice QCD and a vector meson model, respectively. We find that the computed invariant mass spectra (thermal contribution + STAR cocktail) can reproduce the measured ones from STAR at different centralities.Different compared to other model predictions, the obtained elliptic flow of thermal dileptons is larger than the STAR measurement referring to all dileptons. We observe a clear centrality dependence of thermal dilepton not only for elliptic flow v2 but also for higher orders. At a given centrality, vn of thermal dileptons decreases monotonically with n for 2 ≤ n ≤ 5.

Another solution to the puzzle of the direct photon elliptic flow

“…We hope our mechanisms can be repeated by other groups, and more realistic descriptions of the collision system can be obtained. As a prediction of this discrete matter, EPOS3 also provides a quite big elliptic flow of thermal dileptons [20], much bigger than current results obtained by other groups. We hope experimentalists will test with dilepton measurements.…”

“…Thermal photons are produced from the "core" during its expansion and cooling. The treatment of macroscopic variables in order to make thermal photon calculation is quite similar to the sister paper on dileptons [20].…”

Energy, centrality, and momentum dependence of dielectron production at collider energies in a coarse-grained transport approach

“…II A. However, recently the role and importance of viscosity has come into theoretical focus and was studied intensively in hydrodynamical approaches [76][77][78][79][80][81], as it was found that pure ideal hydrodynamic calculations result in an overestimate of the elliptic flow for high transverse momenta and/or wrong slopes for the hadron-p t spectra [68]. With regard to the dilepton emission, the appearance of a shear viscosity might show an effect in two ways: Firstly, by its influence on the bulk evolution-especially an increase of the directed flow and a reduction of the anisotropies-, and secondly by the direct modification of the emission rates due to modifications of the distribution functions [82].…”