26,274 research outputs found
Hadrons in Hot and Dense Matter
The description of excitations in hot and dense (hadronic) matter is
discussed with emphasis on the use of correlation functions as a common
framework for comparing different model (and QCD lattice) calculations with
each other. Typical regimes of applicability of hadronic approaches are
assessed, together with possibilities to confront them with experiment. We also
elaborate on recent developments to relate baryonic in-medium effects to chiral
symmetry restoration.Comment: Convener Talk at the Int. Conf. on Quark Nuclear Physics, June 9-14,
2002 (Juelich, Germany); 3 pages LaTeX including 5 eps-figures and EPJ style
file
Comprehensive Interpretation of Thermal Dileptons at the SPS
Employing thermal dilepton rates based on medium-modified electromagnetic
correlation functions we show that recent dimuon spectra of the NA60
collaboration in central In-In collisions at the CERN-SPS can be understood in
terms of radiation from a hot and dense hadronic medium. Earlier calculated
\rho-meson spectral functions, as following from hadronic many-body theory,
provide an accurate description of the data up to dimuon invariant masses of
about M\simeq 0.9 GeV, with good sensitivity to details of the predicted
\rho-meson line shape. This, in particular, identifies baryon-induced effects
as the prevalent ones. We show that a reliable description of the \rho
contribution opens the possibility to study further medium effects: at higher
masses (M \simeq 0.9-1.5 GeV) 4-pion type annihilation is required to account
for the experimentally observed excess indicating precursor effects of chiral
symmetry restoration (``chiral mixing''), while remaining structures in the
\omega and \phi region are suggestive for modifications in their line shapes as
well.Comment: 4 pages, 4 figures, v2: slightly improved estimate of four-pion
contributions; accepted for publication in Phys. Rev. Let
Measurement of low mass dielectron continuum in sqrt(s_NN)=200GeV Au+Au collisions with the PHENIX experiment at RHIC
Electromagnetic probes are ideally suited to investigate hot and dense matter
produced in high energy heavy ion collisions. They do not undergo strong
interactions and thus probe the time evolution of the collision. The dielectron
continuum is rich in physics. Dalitz decays of light hadrons and direct decays
of vector mesons, which might be modified in the medium, and correlated charm
decays contribute to the spectrum. Furthermore, exotic states, e.g
quasi-particles predicted to exist in the medium only beyond the phase
transition, might leave their footprint in the continuum. Although correlated
e+e- pairs are rare, the 0.24 nb^-1 collected by PHENIX for Au+Au collisions at
sqrt(s_NN)= 200 GeV in 2004 provides a significant sample to investigate the
dilepton continuum. The continuum and its resonances are separated from the
combinatorial background via an event mixing technique. Mass spectra are
presented and compared with the expectations from decays of hadronic sources.Comment: 4 pages, 5 figures, conference proceedin
Dileptons and Medium Effects in Heavy-Ion Collisions
We discuss the status of calculating in-medium modifications of vector-meson
spectral functions in hot and dense matter, their application to dilepton
spectra in ultrarelativistic heavy-ion collisions, and possible relations to
chiral symmetry restoration. We emphasize the importance of constraining
in-medium spectral functions by empirical information from scattering data, QCD
sum rules, and lattice QCD. This is a mandatory prerequisite to arrive at
reliable predictions for low-mass dileptons in heavy-ion collisions.Comment: 8 pages latex, 15 figure
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
Duality and Chiral Restoration from Dilepton Production in Relativistic Heavy-Ion Collisions
We discuss the recent status in the theoretical understanding of dilepton
production in central heavy-ion reactions with the Pb-beam at the full CERN-SpS
energy of 158 AGeV. In the low-mass region (~1 GeV) a strong broadening
of the vector meson resonances in hot and dense matter (especially for the
meson) entails thermal dilepton rates very reminiscent to perturbative
annihilation close to the expected phase boundary of the chiral
symmetry restoring transition. A consistent description of the experimentally
observed enhancement at both low and intermediate masses (1.5 GeV~~3
GeV) in terms of thermal radiation from an expanding fireball can be obtained.Comment: Talk given at ISMD '99 on 'QCD & Multiparticle Production' (Brown
University, Providence, Aug. 9-13), 7 pages LaTeX including 6 ps-/eps-figures
and sprocl.st
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