Dilepton Spectroscopy of QCD Matter at Collider Energies

Ralf Rapp

doi:10.1155/2013/148253

Abstract

Low-mass dilepton spectra as measured in high-energy heavy-ion collisions are a unique tool to obtain spectroscopic information about the strongly interacting medium produced in these reactions. Specifically, in-medium modifications of the vector spectral function, which is well known in the vacuum, can be deduced from the thermal radiation off the expanding QCD fireball. This, in particular, allows to investigate the fate of theρresonance in the dense medium and possibly infer from it signatures of the (partial) restoration of chiral symmetry, which is spontaneously broken in the QCD vacuum. After briefly reviewing calculations of thermal dilepton emission rates from hot QCD matter, utilizing effective hadronic theory, lattice QCD, or resummed perturbative QCD, we focus on applications to dilepton spectra at heavy-ion collider experiments at RHIC and LHC. This includes invariant-mass spectra at full RHIC energy with transverse-momentum dependencies and azimuthal asymmetries, as well as a systematic investigation of the excitation function down to fixed-target energies, thus making contact to previous precision measurements at the SPS. Furthermore, predictions for the energy frontier at the LHC are presented in both dielectron and dimuon channels.

Highlights

The exploration of matter at extremes of temperature (T) and baryon density is at the forefront of research in contemporary nuclear physics, with intimate connections to high-energy, condensed-matter, and even atomic physics [1]
Most of the results shown in the remainder of this article are based on such simplified fireball parametrizations, utilizing the equation of state (EoS) of [42] where a parametrization of lattice QCD (lQCD) results is matched with a hadron resonance gas at Tpc = 170 MeV and subsequent chemical freezeout at Tch = 160 MeV
We have given an overview of medium modifications of the electromagnetic spectral function under conditions expected at collider energies and how these medium effects manifest themselves in experimental dilepton spectra at Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC)

Summary

Introduction

The exploration of matter at extremes of temperature (T) and baryon density (ρB) is at the forefront of research in contemporary nuclear physics, with intimate connections to high-energy, condensed-matter, and even atomic physics [1]. The presented material is partly of review nature, and contains far unpublished results, for example, updates in the use of nonperturbative QGP dilepton rates and equation of state, and detailed predictions for invariantmass and transverse-momentum spectra for ongoing and upcoming experiments at RHIC and LHC, including an excitation function of the beam energy scan program at RHIC.

Thermal Dilepton Rates in QCD Matter

Dilepton Spectra at RHIC and LHC

62.4 GeV 101

Summary and Outlook