Production and Elliptic Flow of Dileptons and Photons in a Matrix Model of the Quark-Gluon Plasma

We consider a nonperturbative approach to the thermal production of dileptons and photons at temperatures near the critical temperature in QCD. The suppression of colored excitations at low temperature is modeled by including a small value of the Polyakov loop, in a “semi”-quark-gluon plasma (QGP). Comparing the semi-QGP to the perturbative QGP, we find a mild enhancement of thermal dileptons. In contrast, to leading logarithmic order in weak coupling there are far fewer hard photons from the semi-QGP than the usual QGP. To illustrate the possible effects on photon and dilepton production in heavy-ion collisions, we integrate the rate with a simulation using ideal hydrodynamics. Dileptons uniformly exhibit a small flow, but the strong suppression of photons in the semi-QGP tends to weight the elliptical flow of photons to that generated in the hadronic phase.

Figure 1

The ratio of the thermal production of dileptons and photons in the semi-QGP versus that in perturbation theory, as a function of temperature. For dileptons, $f_{l\overline{l}}$ from Eq. (3) is for $E=1\mathrm{GeV}$ and $p=0$. For photons, $f_{\gamma }$ in Eq. (7) is independent of the photon momentum. The Polyakov loop $l$ is extracted from the value of the (renormalized) Polyakov loop in lattice QCD [8]; see Ref. [7] for details.

Figure 2

(a) Dilepton yield and (b) elliptic flow computed using music, from the semi-QGP and QGP, plus hadronic matter (HM). This calculation is for Au+Au collisions at the top RHIC energy, $\sqrt{s}=200\mathrm{GeV}/A$, in the 20%–40% centrality class.

Figure 3

(a) Photon yield and (b) elliptic flow using music, from the semi-QGP and QGP, plus hadronic matter. As in Fig. 2, this calculation is for $\mathrm{Au}+\mathrm{Au}$ collisions at the top RHIC energy, in the 20%–40% centrality class.