Formation Time of QGP from Thermal Photon Elliptic Flow

We show that the transverse momentum dependent elliptic flow ν2(pT) of thermal photons is quite sensitive to the initial formation time (τ0) of Quark Gluon Plasma (QGP) for semi-central collision of gold nuclei at RHIC [R. Chatterjee and D. K. Srivastava, Phys. Rev. C 79, 021901(R) (2009)]. 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 ν2. The elliptic flow of thermal photon is dominated by the contribution from the quark matter at intermediate and high pT range and as a result sum ν2 decreases with smaller τ0 for . On the other hand we find that the elliptic flow parameter for hadrons depends only marginally on the value of τ0.