QGP formation time and direct photons from heavy ion collisions

TL;DR

This study uses hydrodynamical models to analyze direct photon data from heavy ion collisions at LHC and RHIC, providing insights into early collision timescales and explaining the large elliptic flow observed.

Contribution

It introduces a method to estimate the formation time of quark-gluon plasma using direct photon data within hydrodynamical models.

Findings

Thermal equilibrium reached in about 1/3 fm/c

Chemical equilibrium reached in about 1.5 fm/c

Predicted higher order flow harmonics for photons

Abstract

We investigated the information carried by the data of direct photons, ie, the transverse momentum spectrum and the elliptic flow $v_{2}$ from Pb+Pb collisions at $\sqrt{s_{NN}}$=2.76TeV measured at Large Hadron Collider (LHC) and from Au+Au collisions at $\sqrt{s_{NN}}$= 200 GeV measured at Relativistic Heavy Ion Collider (RHIC), in the frame work of (3+1)-dimensional ideal hydrodynamical models constrained with hadronic data. We found those direct photon data may serve as a useful clock at the early stage of heavy ion collisions. The time scales of 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, ie, $v_3$, $v_4$ and $v_5$, of direct photons from Pb+Pb collisions at 2.76TeV 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.