Elliptic flow of thermal photons and formation time of quark gluon plasma at RHIC

We calculate the elliptic flow of thermal photons from Au+Au collisions at RHIC energies for a range of values for the formation time $\tau_0$ but fixed entropy (or particle rapidity density). The results are found to be quite sensitive to $\tau_0$. The $v_2$ for photons decreases as $\tau_0$ decreases and admits a larger contribution from the QGP phase which has a smaller $v_2$. The elliptic flow coefficient for hadrons, on the other hand, is only marginally dependent on $\tau_0$.Comment: 2 extra figures and discussion added. To appear in Physical Review C (Rapid Communication

Azimuthal flow of decay photons in relativistic nuclear collisions

An overwhelming fraction of photons from relativistic heavy ion collisions has its origin in the decay of $\pi^0$ and $\eta$ mesons. We calculate the azimuthal asymmetry of the decay photons for several azimuthally asymmetric pion distributions. We find that the $k_T$ dependence of the elliptic flow parameter$v_2$ for the decay photons closely follows the elliptic flow parameter $v_2^{\pi^0}$ evaluated at $p_T \approx k_T+\delta$, where $\delta\approx$ 0.1 -- 0.2 GeV, for typical pion distributions measured in nucleus-nucleus collisions at relativistic energies. Similar results are obtained for photons from the 2-$\gamma$ decay of $\eta$ mesons. Assuming that the flow of $\pi^0$ is similar to those for $\pi^+$ and $\pi^-$ for which independent measurements would be generally available, this ansatz can help in identifying additional sources for photons. Taken along with quark number scaling suggested by the recombination model, it may help to estimate $v_2$ of the parton distributions in terms of azimuthal asymmetry of the decay photons at large $k_T$.Comment: 6 pages, figures added, references added, with more elaborate discussion