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.

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Chiral symmetry restoration and dileptons in relativistic heavy ion collisions

Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions

Hidden local symmetry at loop: A New perspective of composite gauge boson and chiral phase transition

The author has succeeded in writing a well-balanced textbook of the new field of ultra-relativistic heavy-ion collisions (RHICs). The book is primarily suited to theoretically oriented graduate students and researchers (both theorists and experimental physicists). After introducing the necessary kinematical variables and describing the phenomenology of nucleon--nucleon collisions, the author proceeds to elucidate various concepts of high-energy particle physics which are relevant to the study of RHICs. Among these are the hard-scattering model, the Schwinger particle production mechanism, the classical string model and the dual parton model. He then focuses on the concept of the quark--gluon plasma (QGP) and gives an overview of the methods and results of lattice gauge theory. The final chapters of the book are devoted to the creation of a QGP via heavy-ion collisions and its possible signatures. In great detail the author discusses dilepton production, suppression, photon production, Hanbury--Brown--Twiss interferometry and strangeness enhancement. The book is very well written and I can recommend it to all graduate students and researchers interested in the field of RHICs. However, the book has one shortcoming: it focuses on heavy-ion physics in the energy region E > 200 GeV/nucleon and higher. The physics of the hadron gas, important for the later stages of RHICs at CERN and dominant for beam energies in the BEVALAC/SIS/AGS range, is neglected. To acquire an overview of the entire field of RHICs, the reader would profit from a supplementary reading of the more introductory textbook Introduction to Relativistic Heavy-Ion Collisions by L Csernai.

http://iopscience.iop.org/0954-3899/22/1/015/pdf/0954-3899_22_1_015.pdf

Introduction to High-Energy Heavy-Ion Collisions

We study the thermal emission of photons from hot and dense strongly interacting hadronic matter at temperatures close to the expected phase transition to the quark-gluon plasma (QGP). Earlier calculations of photon radiation from ensembles of interacting mesons are reexamined with additional constraints, including new production channels as well as an assessment of hadronic form factor effects. Whereas strangeness-induced photon yields turn out to be moderate, the hitherto not considered $t$-channel exchange of $\ensuremath{\omega}$ mesons is found to contribute appreciably for photon energies above $\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\text{GeV}$. The role of baryonic effects is assessed using existing many-body calculations of lepton pair production. We argue that our combined results constitute a rather realistic emission rate, appropriate for applications in relativistic heavy-ion collisions. Supplemented with recent evaluations of QGP emission, and an estimate for primordial (hard) production, we compute photon spectra at Super Proton Synchrotron (SPS), Relativistic Heavy Ion Collider (RHIC), and Large Hadron Collider (LHC) energies.

Hadronic production of thermal photons

Effects of {ital {bar N}N} polarization on vector meson ({rho} and {omega}) masses at finite temperature are studied. Including a reduced nucleon mass in a hot matter, we find that the reduction of vector meson masses due to the vacuum effect is larger than the increase of their masses from interactions with nucleons and pions in the hot matter. The vector meson masses at finite temperature thus decrease with temperature.

/pdf/effects-of-n-barn-polarization-on-vector-meson-masses-at-1d4oi9ne7o.pdf

Effects of N-barN polarization on vector meson masses at finite temperature.

The properties of pions in hot hadronic matter are analyzed with an effective chiral Lagrangian which includes vector and axial-vector mesons. To obtain the dispersion relation and the absorptive properties of the pions in hot matter, the real and imaginary parts of the pion self-energy are calculated to one-loop order. The dispersion relation is very similar to that of free pions even at [ital T][similar to]160 MeV. The mean free path is estimated from the imaginary part of the self-energy. The mass is obtained from the pole position of the propagator and from the screening limit of the pion self-energy. Even though the deviation of the mass at finite temperature is small, both masses decrease as [ital T] increases. Pions remain massless in the chiral limit at finite temperature. Possible mixing of the pions with the [ital A][sub 1] mesons at finite temperature is discussed.

Pions at finite temperature.

Photon vector-meson coupling and vector meson properties in low temperature pion gas

Thermal effects on dilepton production from π-π annihilation

Dilepton production from hot hadronic matter is studied in an effective chiral Lagrangian with pions, [rho] mesons, and [ital a][sub 1] mesons. We find that the production rates from reactions that involve axial-vector mesons dominate over contributions from all other reactions when the dilepton invariant mass is above 1.5 GeV.

Chungsik Song

Papers

Effects of N-barN polarization on vector meson masses at finite temperature.

Pions at finite temperature.

Photon vector-meson coupling and vector meson properties in low temperature pion gas

Thermal effects on dilepton production from π-π annihilation

Role of the a1 meson in dilepton production from hot hadronic matter.