Polarization of Thermal Dilepton Radiation

TL;DR

This paper predicts polarization patterns of dilepton radiation in high-energy heavy-ion collisions, providing new ways to probe the properties of hot QCD matter through electromagnetic spectral functions and experimental data comparison.

Contribution

It introduces the first predictions of polarization observables in dilepton emission, enabling independent tests of virtual photon selfenergy components in dense QCD matter.

Findings

Low-mass region shows transverse polarization.

High-mass region exhibits unpolarized photons.

Baryon effects induce longitudinal polarization in the rho-meson region.

Abstract

The invariant mass spectra of dileptons radiated from the fireballs formed in high-energy heavy-ion collisions have been successfully used to investigate the properties of hot and dense QCD matter. Using a realistic model for the in-medium electromagnetic spectral function, we predict polarization observables and compare them to experiment. This allows, for the first time, independent tests of the longitudinal and transverse components of the virtual photon's selfenergy. While the low- and high-mass regions exhibit the expected limits of transverse and unpolarized photons, respectively, baryon-driven medium effects in the $\rho$-meson mass region create a marked longitudinal polarization that transits into a largely unpolarized emission from the quark-gluon plasma, thus providing a sensitive test of microscopic emission processes in QCD matter. Applications to available data from the HADES and NA60 experiments at SIS and SPS energies, respectively, are consistent with our predictions and set the stage for quantitative polarization studies at FAIR and collider energies.