Thermal photon $v_2$ with slow quark chemical equilibration

Elliptic flow of direct photons in high-energy heavy-ion collisions has been a topic of great interest since it was experimentally found to be larger than most hydrodynamic expectations. I discuss the implication of possible late formation of the quark component in a hot QCD medium on the photon elliptic flow, because quarks are the source of thermal photons in the deconfined phase. Hydrodynamic equations are numerically solved with the evolution equations for quark and gluon number densities. The numerical results suggest that thermal photon $v_2$ is visibly enhanced by the slow chemical equilibration of quarks and gluons, reducing the aforementioned problem.

Figure 1

Thermal photon elliptic flow $v_2^{\gamma }$ as a function of transverse momentum for a chemically equilibrated medium and a nonequilibrated medium with $c_a=c_c=1.5$ and $c_b=0.2$, 0.5, and 2.0.

Figure 2

Quark number densities in chemical equilibrium and in dynamical evolution with $c_a=c_c=1.5$ and $c_b=0.2$, 0.5, and 2.0 until the local temperature reaches $T_c=0.17$ GeV. The thin lines denote that the system is near the crossover region $T<T_c+\Delta T$ where $\Delta T=0.017$ GeV.

Figure 3

Thermal photon elliptic flow $v_2^{\gamma }$ as a function of transverse momentum for a chemically equilibrated medium and a nonequilibrated medium with $c_b=0.5$ and $c_a=c_c=0$, 1.5, and 3.0.

Figure 4

Thermal photon $p_T$ spectra for a chemically equilibrated and a nonequilibrated medium with $c_a=c_c=1.5$ and $c_b=0.2$, 0.5, and 2.0.