1997 ARGONNE THEORY INSTITUTE - Titles and Abstracts

Reinhard Alkofer (University of Tuebingen)

Temperature Dependence of Eta and Eta' Production in Heavy Ion Collisions and the Physical Origin of the U_A(1) Anomaly

There is no doubt that the U_A(1) anomaly is responsible for the mass of the eta' meson and the mixing between eta and eta'. However, the mechanism(s) leading to a non-vanishing topological susceptibility for the gluonic vacuum is still not resolved. There are successful models for such vacua as, e.g., the Random Instanton Liquid Model, squeezed condensate of gluons or a dynamical eta'-mass from an infrared enhanced gluon exchange. The latter relates the topological susceptibility and therefore the eta'-mass to the string tension. As has been argued some time ago the production rates of eta and eta' in heavy ion collisions depend crucially on the mixing angle and thus on the topological susceptibility. The very weak temperature dependence of the string tension leads thus in this model to almost temperature independent eta and eta' production rates and should make this mechanism experimentally distinguishable from exponentially temperature dependent instanton model predictions. Therefore, heavy ion collisions offer the exciting possibility to clarify the nature of the gluonic component of the QCD vacuum.

Yuki Asakawa (Nagoya University)

What Thermodynamics Tells about the Bulk Properties of the Quark-Gluon Plasma

Revealing the precise nature of the quark-hadron transition is one of the central issues in recent lattice and analytical studies of hot QCD. Furthermore, the behavior of the bulk quantities such as the entropy density s(T), energy density e(T), and pressure P(T) as a function of temperature T is relevant to the formation and evolution of the quark-gluon plasma in the heavy ion collisions planned at BNL and CERN. Lattice QCD calculations have been providing interesting data on the plasma properties above the critical temperature T_c. Among them are some noticable features in QCD simulation with dynamical quarks: (a) P/T^4 approaches the Stefan-Boltzmann limit very slowly as T incerases, (b) e/T^4 has a peak just above T_c and approaches an asymptotic value from above as T increases, and (c) e - 3P not= 0 above T_c, i.e., a deviation from the ideal gas behavior is seen. Some of the above features have been discussed in conjunction with non-perturbative nature of the quark-gluon plasma above T_c. We, however, show that as long as a rapid change of the entropy density occurs across the critical temperature T_c of the QCD phase transition, the pressure and the energy density normalized to their Stefan-Boltzmann values generally deviate from unity for T >> T_c even if there are no interactions among quarks and gluons at T > T_c. In oreder to see this fact, we do not have to refer to the vacuum condensate or the bag constant, but we only need fundamental thermodynamical constraints on the enrtopy density. Furthermore, by making a qualitative comparison with lattice data, we find that most of the behaviors measured in the full lattice QCD simulations can be explained at least qualitatively without introducing the non-perturbative interactions among quarks and gluons at T > T_c.

David Blaschke (University of Rostock)

Hadron Observables and Deconfinement at Finite Temperature and Density

Deconfinement and chiral symmetry restoration are explored in a Dyson-Schwinger equation model of two-flavor QCD at finite temperature. It is shown that, in the chiral limit, both transitions are second order and coincide at $T_c \approx 150$ MeV with the same critical exponent $\beta \approx 0.3$. Within a simpler confining model, the r\^ole of Dirac vector and scalar correlations for the thermodynamic properties of quark matter at the deconfinement transition in the $T-\mu$ plane are demonstrated.
As a consequence of the chiral symmetry restoration transition at finite temperatures and densities, the two-pion decay channel of the scalar $\sigma-$ meson closes what results in the phenomenon of ``critical scattering''. The influence of this effect on photon pair and lepton pair spectra from the hadronisation transition is discussed.

Siu A. Chin (Texas A & M University)

Hamiltonian Lattice Studies of Chiral Meson Field Theories

The latticization of a chiral meson field theory, such as the non-linear sigma model, reduces the field theory to a quantized lattice spin (or rotor) system. While the nature of the chiral phase transition has been studied extensively on Lagrangian lattices, recent interests in heavy-ion collision have focused attention on the pion equation of state and pionic collective modes. These are best studied in the Hamiltonian formulation. In this work, I studied the ground state, as well as the temperature dependence, of the pionic collective modes having quantum numbers of the $\omega$ and $\rho$ mesons. The masses of these mesons have been conjectured to decrease as one approaches the chiral phase transition. I show that, in a Hamiltonian lattice study of the non-linear sigma model, the mass gap of these pionic collective modes indeed vanishes as one approaches the chiral phase transition. This behavior is strikingly similar to the softening of certain type of magnon excitations in spin systems and suggests that such a behavior is, in some sense, "generic".

Pawel Danielewicz (Michigan State Universty)

Hadronic Transport Model with a Phase Transition

George Fai (Kent State University)

The Freeze-Out Problem in Fluid Dynamics

The concept of freeze-out introduces non-thermal matter into the description of nuclear collisions based on fluid-dynamical models, which otherwise assume local thermal equilibrium. Most fluid-dynamics calculations used in heavy-ion physics ignore the effect of freeze-out matter carrying energy and momentum away from the equilibrated fluid. We improve on this approximation by including a coupling between fluid-like and freeze-out parts of the system. The effect of this coupling is demonstrated in a one-dimensional calculation with the equation of state of a massless pion gas. We compare energy-density and velocity profiles with and without interaction between fluid and freeze-out matter. The coupling influences observables such as dilepton spectra.

Charles Gale (McGill University)

Lepton Pair Production in Heavy Ion Collisions and Hadronic Phenomenology

We review some of the effective hadronic Lagrangian techniques used to calculate the production of lepton pairs in heavy ion collisions, in the low and intermediate invariant mass regime. Especially in the latter case, we point out off-shell uncertainties that show up in theoretical estimates and we discuss means of constraining those. As a final application, we consider the finite-temperature mixing of phi and omega, due to scattering from thermal pions.

Sidney Kahana (Brookhaven National Laboratory)

Cascading at Ultrarelativistic Energy: A Means of Converting Two Body Data into Description of Ion-Ion Collisions

Sandra Klevansky (MPI, Heidelberg)

Non Equilibrium Thermodynamics for a Chiral Lagrangian

From lattice gauge simulations of QCD, a chiral phase transition should exist. However, to date there is no clear signal of how this could actually be observed. Experimental procedure is to search in heavy ion collisions for a signal. However, the processes in heavy ion collisions do not have time to equilibrate, and thus a non-equilibrium study of a chiral theory is essential. In this, one may then be able to understand whether divergences in cross sections that occur at phase transitions (such as critical opalescence with light scattering) could lead to observables. We describe a first attempt at constructing such a transport theory for the Nambu-Jona-Lasinio Lagrangian, and are able to derive a Boltzmann-like equation in the lowest order in the double 1/N_c and hbar expansions. It is shown that one can naturally incorporate mesonic degrees of freedom plus the hadronization of quarks and anitquarks into mesons into the field theoretic description. Some simulation results will also be discussed.

Che Ming Ko (Texas A & M University)

Studying Hadron In-Medium Properties in Heavy Ion Collisions

In heavy-ion collisions, a hot and dense hadronic matter is created in the initial stage. This offers the possibility to study the change of hadron properties in medium. I shall review the present status of medium effects in the subthreshold production and collective flow of hadrons as well as the production of low-mass dileptons from heavy ion collisions.

Volker Koch (Lawrence Berkeley Laboratory)

The Lambda (1405) in Matter

We adress the longstanding question about the nature of the Lambda(1405) resonance; is it a K^- N bound state or rather a genuine three quark state? We will argue that the bound state picture naturally explains the observed attractive optical potential of the K^- in matter. It also leads to a considerable beter description of stopped K^- experiments. Possible new and even more conclusive experiments will be discussed.

Dimitri Kusnezov (Yale University)

Field Theory far from Equilibrium

One of the important questions concerning RHIC is to what extent non-equilibrium physics will be important. We have been dynamical aspects of field theory both near and far from equilibrium, and the effects of the departure from equilibrium on the transport coefficients. Some prelimenary results will be presented on non-equilibrium steady states and phase transitions in such environments.

Kurt Langfeld (University of Tuebingen)

A New State of Hadronic Matter at High Density

We propose in this article that if the chemical potential exceeds a critical value in dense hadronic medium, a first-order phase transition to a new matter state with Lorentz symmetry spontaneously broken (in addition to the explicit breaking) takes place. As a consequence, light vector mesons get exited as `almost' Goldstone bosons and the Pi^+ Pi^- annihilation process will be suppressed. Since the light vector mesons dominantly couple to photons, the presence of these new vector mesons could lead to an enhancement in the dilepton production at an invariant mass lower than the free-space vector-meson mass. We provide a low-energy quark theory from which it becomes clear that the above scenario is the generic case for quark theories with a strong interaction in the vector channel. We discuss possible relevance of this phase to the phenomenon of the enhanced dilepton production in relativistic heavy-ion collisions in the invariant mass range of 300... MeV as recently observed in CERES and HELIOS experiments.

Pieter Maris (Argonne National Laboratory)

A Continuum Study of Deconfinement at Finite Temperature and Chemical Potential

The Dyson-Schwinger equations [DSEs] provide a Poincar'e-invariant, nonperturbative, renormalisable, continuum framework for analysing QCD. The applications of the DSEs to meson phenomenology are demonstrated in a confining, renormalisable, DSE-model of QCD, defined by the choice of a gluon propagator. The parameters are fixed by fitting a range of pi and rho meson observables at T=0 and the extension to finite temperature involves no new parameters. A continuum order parameter for deconfinement is introduced and employed to establish that, in the chiral limit, deconfinement and chiral symmetry restoration are coincident at T_c approx. 150MeV. The transitions are second order and each has the same critical exponent: beta approx. 0.3. The DSE framework allows a straightforward extension to finite quark chemical potential. For any nonzero quark chemical potential, the deconfinement and chiral symmetry restoration transitions are first order.

Ulf-G. Meissner (FA Juelich)

Chiral Dynamics: a Status Report

I review the present status of chiral dynamics. I address in particular the role of pion--pion scattering to pin down the Standard Model parameters related to the chiral symmetry breakdown and the effects of virtual photons. Some remarks about Goldstone boson properties at finite temperature and density are also made.

Joelle Murray (Michigan State University)

e+ e- Pairs at CERN SPS

We introduce a new hadronic event generator for the simulation of CERN SPS, RHIC, and LHC heavy ion collisions. As a first application, we calculate di-lepton spectra and compare to the measured spectra from the CERES and HELIOS collaborations. We find that our results for primary produced particles agree well with other event generators and are significantly underprediucting the data. However, inclusion of secondary hadronic processes leads to better agreement.

Mannque Rho (CE Sacley)

An Effective Chiral Lagrangian for Dense Hadronic Matter

Fluctuations in various flavor directions in dense hadronic matter are described in terms of an effective chiral Lagrangian with BR scaled parameters. Mapping between mean-field chiral Lagrangian theory and Landau Fermi-liquid fixed point theory leads to some remarkable relations between the quantities that figure in low-energy nuclear spectroscopic properties (such as orbital gyromagnetic ratios, giant dipole resonances) and the behavior of strange and non-strange hadrons in hot and dense medium as probed in relativistic heavy-ion collisions.

Stefan Schramm (GSI, Darmstadt)

QCD and Instantons at Finite Temperature on the Lattice

The current status of lattice QCD calculations at finite temperature is discussed. Special emphasis is laid on the effect of instanton contributions and their behavior with changing temperature. Numerical results of quenched and unquenched calculations are presented.

Heinz Sorge (SUNY, Stony Brook)

Signals from the QCD Phase Transition in AA Collisions: Critical Opalescence and Soft Expansion

It can be expected that the transition from a quark-gluon plasma into a gas of hadrons is close to a second order phase transition. In this case the mass of the sigma meson which serves as an order parameter on the hadronic side will drop in the transition region. I discuss repercussions of a dropping sigma mass on the dynamics in ultrarelativistic nucleus-nucleus collisions. In particular, critical opalescence may occur and reflect itself in the final pion correlation functions. Furthermore, I discuss the sensitivity of flow observables on a `softening' of the pressure due to the vicinity of the quark-hadron transition.

Madeleine Soyeur (CE Saclay)

Production and Propagation of Vector Mesons in Nuclei near Threshold

We discuss the production of rho- and omega-mesons off protons and off nuclear targets in reactions induced by photons and by pions. We concentrate on the kinematics close to the production threshold (E<3 GeV). We present recent results on rho- and omega-meson propagation in nuclear matter.

Markus Thoma (University of Giessen)

Photons and Dileptons from the Quark-Gluon Plasma

The production rates of energetic photons and dileptons from a quark-gluon plasma at finite temperature and chemical potential are calculated using the resummation technique of Braaten and Pisarski. Non-equilibrium effects are discussed in addition.

Jochen Wambach ( GSI, Darmstadt)

Low-Mass e^+e^-Pairs from In-Medium Rho-Meson Propagation

Based on a realistic description of the \rho-meson in free space, it's medium modifiaction in a hot hadron gas, generated by hadronic rescattering processes, will be dicussed. Within the vector dominance model, the resulting in-medium \rho spectral function is applied to calculate e^+e^- spectra as recently measured in relativistic heavy-ion collisions at CERN-SpS energies in the CERES experiment. The model is able to largely explain the observed dielectron yields for invariant masses below 1 GeV.

Xin-Nian Wang (Lawrence Berkeley Laboratory)

Studying Chiral Phase Transition at RHIC

In the chiral limit, the relaxation of a chiral symmetrical phase could lead to a disoriented chiral condensate with nonvanishing values in the pion direction. However, the explicit chiral symmetry breaking due to small but finite quark masses makes this impossible. Instead, a nonequilibrium phase transition can lead to the amplification of the soft pion mode. Such amplification is demonstrated in a linear-sigma model in the classical approximation. The consequences of such amplification such as soft pion production and photon and dilepton production will be discussed.

Wolfgang Weinhold (GSI, Darmstadt)

Thermodynamics with Resonant States

We discuss a consistent approach to the thermodynamics of hadronic matter including resonances. As an example we consider an interacting system of pions and nucleons including the pi N P_(33) channel. In the low-density limit, the leading term of the virial expansion of the thermodynamic potential is recovered. We present an instructive diagrammatic interpretation of the contributions to the total baryon number. The generalization to resonances with several decay channels is also given.
Finally, we examine the consequences for the low-mass enhancement of the pi spectra in heavy-ion collisions at intermediate energies.

Wolfram Weise (Technical University of Munich)

Pseudoscalar and Vector Mesons in a Nuclear Environment

A summary is given on recent developments related to the following topics:
1) Quark condensate in dense nuclear matter
2) In-medium effective masses of pions and kaons, with contact to experiments on deeply bound pionic states and subthreshold kaon production at GSI
3) Rho, omega and phi meson spectra in matter and QCD sum rules.

Urs Wiedemann (University of Regensburg)

Reconstruction of the Final State in Heavy Ion Collisions

The first part of my presentation reviews in a model-independent framework which information about the spatial extension, expansion velocity and emission duration of a heavy ion collision can be extracted in a Hanbury-Brown/Twiss (HBT) interferometric analysis of 2-particle correlation functions. I focus on the spatio-temporal interpretation of Gaussian HBT radius parameters (both the Cartesian and the Yano-Koonin-Podgoretskii ones) and I explain how the method of q-variances can be used to quantify deviations of the correlator from a Gaussian shape. In general, the collective expansion of the source leads to a distinct pair momentum dependence of HBT radius parameters. This dependence however can be modified considerably by resonance decay contributions. In the study of a model which assumes local thermalization at freeze-out and produces hadronic resonances by thermal excitation, we illustrate how different geometrical and dynamical source characteristics are reflected in the momentum dependence of the correlator. Also, we discuss in detail how scenarios with and without transverse flow can be disentangled with the help of higher order q-variances.

Bolek Wyslouch (MIT, Cambridge)

Experimental Searches for Disoriented Chiral Condensates

Ismail Zahed (SUNY, Stony Brook)

Dilepton and Photon Emission From a Hadronic Gas

I will use a newly formulated set of Ward identities for broken chiral symmetry in QCD, to analyse dilepton and photon emission from a hadronic gas at finite temperature and density. Using a simple model for the space-time evolution, I will discuss the results in light of the WA80 and CERES data.