Measuring the isotropization time of quark-gluon plasma from direct photons at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

TL;DR

This paper investigates the isotropization time of quark-gluon plasma at RHIC by analyzing direct photon data, finding it to be around 1 fm/c and relatively insensitive to initial conditions, thus providing insights into early plasma dynamics.

Contribution

It introduces a method to extract the isotropization time of QGP from photon spectra, considering initial anisotropy and flow effects, improving understanding of plasma equilibration.

Findings

Isotropization time is about 1 fm/c.

Initial anisotropy improves photon data description.

Results are robust against initial condition variations.

Abstract

We calculate transverse momentum distribution of direct photons from various sources by taking into account the initial state momentum anisotropy of {\em Quark-Gluon-Plasma} (QGP) and late stage transverse flow effects. The total photon yield, calculated for various combinations of initial conditions, transition temperatures, is then compared with the recent measurement of photon transverse momentum distribution by the PHENIX collaboration. It is shown that the presence of such an anisotropy can describe the PHENIX photon data better than the isotropic case. We show that the isotropization time thus extracted remains of the order of 1 fm/c and is almost insensitive to the initial conditions used.