Study the early stage of heavy ion collisions with direct photons

Based on the modelling of the collective motion in AuAu collisions at √S_NN = 200 GeV at centrality 0-20% and 20-40% and PbPb collisions at √S_NN = 2.76 TeV at centrality 0-40% with a 3+1D event-averaged ideal hydrodynamics constrained with hadronic data, we study the transverse momentum spectrum and elliptic flow of direct photons and find that the recent direct photon data from both PHENIX collaboration at RHIC and ALICE collaboration at LHC favour an early beginning of collective expansion (τ₀ < 0.6 fm/c) and a late formation of quark gluon plasma (τ ~ 2 fm/c). (author)