QGP formation time and direct photons from heavy ion collisions

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.

6 pages, 5 figures

References in corpus (8)

• The Color Glass Condensate
• Conformal anomaly as a source of soft photons in heavy ion collisions
• Mass ordering of differential elliptic flow and its violation for phi mesons
• Measurement of Direct Photons in Au+Au Collisions at sqrt(s_NN) = 200 GeV
• Measurement of Direct-Photon Elliptic Flow in Pb-Pb Collisions at sqrt(NN) = 2.76 TeV
• Anisotropy of photon production: Initial eccentricity or magnetic field
• Elliptic Flow from a Hybrid CGC, Full 3D Hydro and Hadronic Cascade Model
• Centrality-dependent direct photon pt spectra in Au+Au collisions at RHIC

Cited by in corpus (58)

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