Quark-gluon plasma formation time and direct photons from heavy ion collisions

Fu-Ming Liu and Sheng-Xu Liu

Phys. Rev. C 89, 034906 – Published 13 March 2014

Abstract

We investigated the information carried by the data on direct photons, $i . e .$ , the transverse momentum spectrum and the elliptic flow $v_{2}$ from Pb + Pb collisions at $\sqrt{s_{N N}} = 2.76$ TeV measured at the Large Hadron Collider and from Au + Au collisions at $\sqrt{s_{N N}} = 200$ GeV measured at the Relativistic Heavy Ion Collider, in the framework of ( $3 + 1$ )–dimensional ideal hydrodynamical models constrained with hadronic data. We found that these direct photon data may serve as a useful clock at the early stage of heavy ion collisions. The time scales for reaching thermal and chemical equilibrium, extracted from those data, are about 1/3 and 1.5 fm/ $c$ , respectively. Thus the large elliptic flow of direct photons is explainable. High-order harmonics, $i . e .$ , $v_{3}$ , $v_{4}$ , and $v_{5}$ , of direct photons from Pb + Pb collisions at 2.76 TeV are also predicted, as a further test to compete with those who claim new sources of photons to account for the large elliptic flow of direct photons.