Nuclear modification factor in an anisotropic {\em Quark-Gluon-Plasma}

TL;DR

This paper investigates how initial momentum anisotropy in the quark-gluon plasma affects the nuclear modification factor $R_{AA}$ in heavy ion collisions, using a phenomenological model to compare with experimental data and estimate isotropization times.

Contribution

It introduces a model incorporating initial state momentum anisotropy to analyze $R_{AA}$ and constrains the isotropization time scale using experimental data from RHIC and LHC.

Findings

Estimated isotropization time $ au_{iso}$ between 0.5 and 1.5 fm/c.

Model sensitivity to initial conditions discussed.

Extension of $R_{AA}$ predictions to LHC energies at 5.5 TeV.

Abstract

We calculate the nuclear modification factor ($R_{AA}$) of light hadrons by taking into account the initial state momentum anisotropy of the quark gluon plasma (QGP) expected to be formed in relativistic heavy ion collisions. Such an anisotropy can result from the initial rapid longitudinal expansion of the matter. A phenomenological model for the space time evolution of the anisotropic QGP is used to obtain the time dependence of the anisotropy parameter $\xi$ and the hard momentum scale, $p_{\rm hard}$. The result is then compared with the PHENIX experimental data to constrain the isotropization time scale, $\tau_{\rm iso}$ for fixed initial conditions (FIC). It is shown that the extracted value of $\tau_{\rm iso}$ lies in the range $0.5 \leq \tau_{\rm iso} \leq 1.5$. However, using fixed final multiplicity (FFM) condition does not lead to any firm conclusion about the extraction of the isotropization time. The present calculation is also extended to contrast with the recent measurement of nuclear modification factor by ALICE collaboration at $\sqrt{s}=2.76$ TeV. It is argued that in the present approach, the extraction of $\tau_{\rm iso}$ at this energy is uncertain and, therfore, refinement of the model is necessary. The sensitivity of the results on the initial conditions has been discussed. We also present the nuclear modification factor at LHC energies with $\sqrt{s} = 5.5$ TeV.