Formation Time of QGP from Thermal Photon Elliptic Flow

We show that the transverse momentum dependent elliptic flow $v_2(p_T)$ of thermal photons is quite sensitive to the initial formation time ($\tau_0$) of Quark Gluon Plasma (QGP) for semi-central collision of gold nuclei at RHIC \cite{tau}. A smaller value of the formation time or a larger initial temperature leads to a significant increase in the thermal photon radiation from QGP phase, which has a smaller $v_2$. The elliptic flow of thermal photon is dominated by the contribution from the quark matter at intermediate and high $p_T$ range and as a result sum $v_2$ decreases with smaller $\tau_0$ for $p_T \ge 1.5$ GeV. On the other hand we find that the elliptic flow parameter for hadrons depends only marginally on the value of $\tau_0$.