Hydrodynamic Description of Dilepton Production

TL;DR

This thesis investigates thermal dilepton production in heavy-ion collisions, emphasizing the effects of viscosity, chiral symmetry restoration, and hydrodynamic evolution, with comparisons to experimental data from NA60 and PHENIX.

Contribution

It introduces a comprehensive framework combining chiral symmetry constraints, viscous hydrodynamics, and dilepton rate calculations, advancing understanding of QGP properties and thermalization mechanisms.

Findings

Viscous effects significantly alter dilepton spectra and elliptic flow.

Shear viscosity influences the invariant mass and transverse momentum distributions.

Comparison with experimental data constrains thermalization time and viscosity-to-entropy ratio.

Abstract

The first part of this thesis focuses on the production of thermal dileptons from a hadronic gas at finite temperature. The rates are calculated by an expansion in Pion density and constrained by broken chiral symmetry and vacuum correlation functions, many of which have been measured by experiment. We focus on emission processes having two Pions in the final state. Next, follows a separate discussion on viscous hydrodynamics and its effect on $p_\perp$ spectra and elliptic flow. A non-central hydrodynamic model of Au-Au collisions in 2+1 dimensions is simulated.Off-equilibrium corrections to the distribution can bring about large changes in the differential elliptic flow, especially at higher $p_\perp$. Also discussed is the shear viscous correction to dilepton production in a quark-gluon plasma (QGP) emanating from $q\overline{q}$ annihilation in the Born approximation. It is argued that a thermal description is reliable for invariant masses less than $M_{max} \approx (2\tau_0 T_0^2)/(\eta/s)$. Shear viscosity leads to qualitative differences in dilepton $p_\perp$ spectrum, which could be used to extract information on the thermalization time, viscosity to entropy ratio and possibly the thermalization mechanism in heavy-ion collisions. Finally, the dilepton rates used in this work are integrated over the space-time evolution of the collision region and compared to the recent results from the NA60 experiment at CERN and the PHENIX experiment at RHIC. The role played by chiral symmetry restoration in the hadronic phase and viscosity in the QGP phase is discussed.