5,685 research outputs found
Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions
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.Comment: 164 pages LaTeX including 88 eps-/ps-figures, Review Article to
appear in Adv. Nucl. Phy
Probing Chiral Symmetry Restoration with Heavy Ions
It is discussed how chiral symmetry restoration manifests itself through
mixing of vector and axial-vector correlators. The vector correlator is
directly accessible in relativistic heavy-ion collisions. Within models of the
vector correlator its implications for low-mass dilepton spectra are reviewed.Comment: 11 pages LaTeX, incl. 6 eps-figures and appb.sty; Talk given at the
Workshop on 'The Structure of Mesons, Baryons and Nuclei', Cracow, May 1998,
in honor of J. Speth's 60th birthday, to be published in Acta Physica
Polonica
Quark Recombination and Heavy Quark Diffusion in Hot Nuclear Matter
We discuss resonance recombination for quarks and show that it is compatible
with quark and hadron distributions in local thermal equilibrium. We then
calculate realistic heavy quark phase space distributions in heavy ion
collisions using Langevin simulations with non-perturbative T-matrix
interactions in hydrodynamic backgrounds. We hadronize the heavy quarks on the
critical hypersurface given by hydrodynamics after constructing a criterion for
the relative recombination and fragmentation contributions. We discuss the
influence of recombination and flow on the resulting heavy meson and single
electron R_AA and elliptic flow. We will also comment on the effect of
diffusion of open heavy flavor mesons in the hadronic phase.Comment: Contribution to Quark Matter 2011, submitted to J.Phys.G; 4 pages, 5
figure
Rho Meson Propagation and Dilepton Enhancement in Hot Hadronic Matter
A realistic model for the free rho meson with coupling to two-pion states is
employed to calculate the rho propagator in a hot and dense hadron gas. The
medium modifications are based on hadronic rescattering processes: intermediate
two-pion states are renormalized through interactions with surrounding nucleons
and deltas, and rho meson scattering is considered off nucleons, deltas, pions
and kaons. Constraints from gauge invariance as well as the full off-shell
dynamics of the interactions are accounted for. Within the vector dominance
model we apply the resulting in-medium rho spectral function to compute
production rates from annihilation. The calculation of
corresponding spectra as recently measured in central collisions of
heavy-ions at CERN/SpS energies gives reasonable agreement with the
experimental data.Comment: 27 pages RevTeX, 9 eps-figures, submitted to Nucl. Phys.
Medium Modifications of the Rho Meson at CERN/SPS Energies
Rho meson propagation in hot hadronic matter is studied in a model with
coupling to states. Medium modifications are induced by a change of
the pion dispersion relation through collisions with nucleons and in
the fireball. Maintaining gauge invariance dilepton production is calculated
and compared to the recent data of the CERES collaboration in central S+Au
collisions at 200 GeV/u. The observed enhancement of the rate below the rho
meson mass can be largely accounted for.Comment: 10 pages RevTeX and 2 figures (uuencoded .ps-files
A Microscopic Calculation of Photoabsorption Cross Sections on Protons and Nuclei
A recently developed model for -meson propagation in dense hadronic
matter is applied to total photoabsorption cross sections in -proton
and -nucleus reactions. Within the vector dominance model the photon
coupling to the virtual pion cloud of the nucleon, two-body meson-exchange
currents, as well as -nucleon resonances are included. Whereas the
reaction is determined by the low-density limit of the model, higher
orders in the nuclear density are important to correctly account for the
experimental spectra observed on both light and heavy nuclei over a wide range
of photon energies, including the region below the pion threshold. In
connection with soft dilepton spectra in high-energy heavy-ion collisions we
emphasize the importance of photoabsorption to further constrain the parameters
of the model.Comment: 10 pages RevTeX, 2 eps-figure
Momentum Dependence of the Pion Cloud for Rho Mesons in Nuclear Matter
We extend hadronic models for rho-meson propagation in cold nuclear matter
via coupling to in-medium pions to include finite three-momentum. Special care
is taken to preserve gauge invariance. Consequences for photoabsorption on the
proton and on nuclei as well as for the dilepton production in relativistic
heavy-ion collisions are discussed.Comment: 32 pages, 18 figures. Corrected version, accepted for publication in
Nucl. Phys.
Modifications of the Rho Meson from the Virtual Pion Cloud in Hot and Dense Matter
The modification of the rho-meson self-energy due to the coupling to
in-medium pions is calculated consistently at finite baryon density and
temperature, keeping the full 3-momentum dependence in a gauge invariant way.
As a function of nucleon density, the rho-meson spectral function is strongly
enhanced in the invariant mass region M < 650 MeV, while the maximum, i.e. the
pole mass, is slightly shifted upwards. As a function of temperature, for fixed
nucleon density, the imaginary part of the self-energy increases further due to
Bose-enhancement. At the same time the mass shift from the real part becomes
very large. As a consequence of these medium effects, the dilepton rate in the
low-mass region M < 650 MeV increases strongly, while the peak at M = 770 MeV
disappears.Comment: 18 pages, 9 figures; resonance contributions adde
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