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.