Thermal dilepton polarization and dynamics of the QCD plasma in relativistic heavy-ion collisions

TL;DR

This paper investigates how the polarization of dileptons produced in high-energy heavy-ion collisions can serve as a sensitive probe of the early thermalization process of the quark-gluon plasma, using advanced NLO calculations.

Contribution

It provides the first theoretical analysis of dilepton polarization in Pb+Pb collisions at LHC energies, highlighting the importance of NLO effects and the potential of dileptons to probe early plasma dynamics.

Findings

Intermediate invariant-mass dileptons probe thermal equilibration.

NLO and LO polarization results differ significantly.

Dilepton polarization offers insights beyond hadronic observables.

Abstract

We present the first theoretical study of the polarization of lepton pairs produced in $\sqrt{s_\mathrm{NN}} = 5.02$ TeV Pb+Pb collisions at the LHC, using next-to-leading order (NLO) dilepton emission rates. These calculations employ a multi-stage framework to simulate the evolution of relativistic heavy-ion collisions, and to explore the sensitivity of polarization to early times. It is found that the intermediate invariant-mass dileptons are indeed probes of the thermal equilibration process, and go beyond the reach of hadronic observables. We compute the polarization anisotropy coefficient obtained with LO dilepton rates, and show that the LO and NLO results differ radically, both in trend and in magnitude, at low and intermediate lepton pair invariant masses.