Another solution to the puzzle of the direct photon elliptic flow

The observed large elliptic flow of direct photons is a puzzle in relativistic heavy ion physics. Our previous work provided a possibility to this puzzle with the delayed formation of the quark gluon plasma (QGP) in relativistic heavy ion collisions. In this work, we got the measured transverse momentum spectra, elliptic flow v2 and triangular flow v3 of direct photons from Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) energy \sqrt s_NN = 200 GeV with all centrality classes explained at the same time without any additional parameter to EPOS3, a hydrodynamic model which can successfully reproduce hadronic data such as rapidity distributions, transverse momenta, elliptic flows and triangular flows from variuos collision systems. The key point is that, EPOS3 has an initial space eccentricity similar to other models, however, a stronger radial flow and a weaker momentum eccentrcity made a good description to the flows of both hadrons and direct photons. The created QGP matter seems discrete than a soup.Comment: 7 pages 8 fig

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

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 $\sqrt{s_{NN}}=200$~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 $v_{2}$ and $v_{3}$ !) 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 $v_{2}$ but also for higher orders. At a given centrality, $v_{n}$ of thermal dileptons decreases monotonically with $n$ for $2\leq n\leq5$.Comment: 10pages, 12fig