[Paper Review] Dilepton Spectroscopy of QCD Matter at Collider Energies

Ralf Rapp|arXiv (Cornell University)|Apr 8, 2013

High-Energy Particle Collisions Research12 references4 citations

TL;DR

This paper investigates dilepton spectroscopy as a probe of QCD matter at collider energies, using thermal dilepton emission rates from hot quark-gluon and hadronic phases to study in-medium modifications of the $ ho$ meson spectral function. Key findings show that low-mass dilepton spectra are sensitive to chiral symmetry restoration and QGP collectivity, with predicted hardening of transverse-momentum spectra and enhanced yields at LHC energies, providing a 'QGP barometer' for fireball dynamics.

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 the $ρ$ resonance 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.

Motivation & Objective

• To investigate how dilepton spectra at collider energies (RHIC and LHC) can reveal in-medium modifications of the $ ho$ meson spectral function.
• To assess the role of chiral symmetry restoration in the QCD phase diagram via dilepton emission from the hot, dense medium.
• To connect theoretical predictions of dilepton emission rates with experimental observables such as invariant-mass spectra, transverse-momentum distributions, and azimuthal anisotropies.
• To explore the excitation function of dilepton yields from SPS to LHC energies, testing for signatures of critical slowing down near $T_{\rm pc}$.
• To evaluate the sensitivity of dilepton observables to QGP collectivity and fireball lifetime, using the LMR as a probe of early-stage dynamics.

Proposed method

• Utilizes effective hadronic theory, lattice QCD, and resummed perturbative QCD to compute thermal dilepton emission rates from hot QCD matter.
• Applies a relativistic transport model with a crossover equation of state (lattice QCD-based) to simulate fireball evolution and emission from both QGP and hadronic phases.
• Computes invariant-mass spectra, transverse-momentum distributions, and elliptic flow ($v_2$) of dileptons, accounting for time-dependent temperature and collective expansion.
• Compares emission rates from QGP and hadronic phases near $T_{\rm pc} \simeq 160$ MeV, emphasizing the near-degeneracy of rates in the transition region.
• Analyzes the dependence of dilepton yields on charged-particle rapidity density ($N_{\rm ch}^\alpha$) to infer fireball lifetime and potential critical slowing down.
• Evaluates the impact of heavy-flavor decays on invariant-mass spectra in the intermediate-mass region (IMR), distinguishing prompt dileptons from correlated heavy-quark contributions.

Experimental results

Research questions

• RQ1 How do in-medium modifications of the $ ho$ meson spectral function manifest in low-mass dilepton spectra at RHIC and LHC energies?
• RQ2 To what extent can dilepton spectra serve as a probe of chiral symmetry restoration in the QCD medium at $μ_B \simeq 0$?
• RQ3 What is the energy dependence of dilepton yields and transverse-momentum spectra, and how does it reflect QGP collectivity and fireball lifetime?
• RQ4 Can the excitation function of dilepton yields from SPS to LHC reveal signatures of critical slowing down near the crossover transition?
• RQ5 How do dilepton observables in the IMR compare to those in the LMR in probing early QGP temperatures and emission dynamics?

Key findings

• The LMR dilepton spectra at RHIC and LHC are dominated by hadronic emission near the $ρ$ mass, even though the QGP fraction increases with collision energy due to the lattice QCD-based equation of state.
• Transverse-momentum spectra in the LMR are predicted to harden significantly with energy, with inverse-slope parameters increasing from ~220 MeV at SPS to ~450 MeV at LHC-5500, reflecting enhanced QGP collectivity.
• The total yield scaling with charged-particle rapidity density follows $N_{\rm ch}^\alpha$ with $\alpha \simeq 1.8$ from RHIC to LHC, indicating a strong dependence on fireball lifetime.
• A non-monotonous behavior in the excitation function could signal critical slowing down near $T_{\rm pc}$, making dilepton enhancement a potential signature of a critical point.
• The QGP contribution to LMR emission becomes dominant in the transition region near $T_{\rm pc}$, making it a key probe of chiral restoration in a regime accessible to lattice QCD.
• IMR spectra remain promising for direct temperature measurements, provided correlated heavy-flavor decays can be reliably subtracted or modeled.