Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions

Abstract

The current theoretical status in the analysis and interpretation of low-mass dilepton measurements in (ultra-) relativistic heavy-ion experiments is reviewed. Special emphasis is put on potential signals of (partial) restoration of dynamically broken chiral symmetry in a hot and dense hadronic medium. It follows from chiral symmetry alone that parity partners of hadronic correlation functions must become identical when the symmetry is restored. The assessment of medium effects in the vector channel, which governs the dilepton production, thus necessitates a simultaneous treatment of the vector and axialvector degrees of freedom. While significant progress in this respect has been made some open questions remain in establishing a rigorous link in the mass region below 1 GeV. From the present calculations a suggestive 'quark-hadron duality' emerges near the phase boundary. It implies substantial medium effects in the dilepton signal from the hadronic phase which smoothly matches a perturbative description within the plasma phase.

Thermodynamics of strong-interaction matter from lattice QCD

“…The dissociation of heavy quark bound states has long been advocated as one of the striking signatures for the formation of a deconfined color screened medium 18 while the melting of light quark bound states is intimately related with the restoration of chiral symmetry. 19 We will report on the status of calculations of transport coefficients and in-medium hadron properties in Sections 6 and 7.…”

Nucleons and parity doubling across the deconfinement transition

“…In the past decades, chiral symmetry restoration at finite temperature has been studied in great detail in the mesonic sector [3]. One reason is that mesonic correlation functions are relatively easily accessible on the lattice [4,5].…”

Dyson-Schwinger Equations: Density, Temperature and Continuum Strong QCD

“…In approaches based on elementary hadronic degrees of freedom; e.g., those reviewed in Ref. [239], this consideration is bypassed. Colour singlet baryon density is introduced directly via the expedient of in-medium elementary meson and nucleon propagators, which are then employed in calculating the myriad nuclear-matter many-body loop integrals that contribute to observable processes.…”

“…Adequate explanations are found in: 1) collisional broadening; i.e., the ρ has a shorter lifetime in an hadron-rich medium, and an increase in the ρ-meson's width due to in-mediummodified hadron-loop contributions to the ρ-meson's self-energy [239,249]; and 2) a simple reduction in the ρ-meson mass [249,250]. The phenomenological models require large values of temperature [T ∼ 0.15 GeV] and baryon density [ρ ∼ 1-2 ρ 0 ] to describe the data.…”